JPH10170658A - X-ray image pick-up device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To set a threshold voltage higher than voltage of normal operation region and reduce leak current. SOLUTION: A photoelectric converting part 5 converts incident X-ray to charge, an accumulative capacity part 6 accumulates the converted charge, TFT (T hin Film Transistor) 14 reads the accumulated charge, and TFT 7, whose one end is connected to the accumulative capacity part 6, sweeps out charge accumulated in the aecumulative capacity part 6 when impressed voltage is higher than a threshold voltage. Thus, the X-ray image pick-up device can change its threshold voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to an X-ray imaging device that converts a line image into an image.

[0002]

2. Description of the Related Art In an X-ray imaging apparatus, there is a method of obtaining an X-ray image by combining an image intensifier (II) for converting X-rays into light and a television apparatus (II-). TV system). This method uses an image
The size of the X-ray input surface of the intensifier is the size that can be imaged, and there are some with a field of view of about 16 inches.

An X-ray image converted into light is once formed by an image intensifier output unit. This output image is picked up by a television camera via an optical system and output as an electric image. . In this method, an X-ray image can be observed in real time.

However, this method has disadvantages such as poor resolution and a large imaging system as compared with a film system.

In recent years, it has a high resolution characteristic, I. -T
As a next-generation X-ray imaging device with the real-time characteristics of the V system, thin film transistor
or, hereinafter, referred to as TFT. An X-ray flat panel detector using ()) as a switching gate has been proposed.

This X-ray flat panel detector has a pixel portion comprising a semiconductor layer for directly converting X-rays into electric charges on a detection surface on a flat plate,
It is composed of a charge storage section composed of a capacitor for storing charges, and a read switch (referred to as a read SW) for reading the charges.

An X-ray imaging apparatus having this X-ray flat panel detector is a conventional I.D. I. -As in the case of a TV device, the image display is excellent in instant displayability, and electrical image storage is easy. Furthermore, it is structurally thin.

However, in the X-ray flat panel detector, the charge increases in proportion to the amount of X-ray incident on each pixel, and a high voltage may be applied to the charge storage unit. This applied voltage is applied to the input side of the readout SW during X-ray irradiation.

However, if the applied voltage exceeds a certain voltage during X-ray irradiation, the read SW may be destroyed.

[0010] This situation is considered to occur particularly when excessive X-rays are irradiated to the detector. Clinically, for example, when X-rays emitted from an X-ray tube do not pass through the human body as the subject and directly enter the detector, or when X-rays are erroneously exposed for a long time. For example, the charge increases, and a high voltage is applied to the charge storage unit. Therefore, there is a problem that the voltage applied to the input side of the read SW exceeds a certain voltage, and the read SW is destroyed.

Therefore, the present applicant has solved this problem and filed an unpublished Japanese Patent Application No. Hei 8-161977 on June 21, 1996. The X-ray flat panel detector described in Japanese Patent Application No. 8-161977 has a charge conversion unit, a charge storage unit, a charge reading unit such as a reading SW, a Zener diode, or a thin film transistor (T
FT) or the like.

The charge conversion unit converts the incident X-rays, which are provided corresponding to each of a plurality of pixels arranged on the detection surface, into charges. The charge storage unit is provided corresponding to each charge conversion unit and stores the charge converted by the charge conversion unit.

The charge reading section is provided corresponding to each charge storage section and reads out the charges stored by the charge storage section. The sweeping section is provided corresponding to each charge reading section,
Connected to the input side of the charge readout unit.

Each sweeping section sweeps out the electric charge accumulated in the electric charge accumulating section when the applied voltage becomes equal to or higher than a predetermined voltage lower than the voltage at which the electric charge reading section is destroyed. That is, before the applied voltage reaches the potential at which the charge readout portion is destroyed, the charges are swept out using the voltage breakdown characteristics of the sweeping portion, so that a high voltage due to the charges is applied to the charge readout portion during X-ray irradiation. Even when the voltage can be applied, the charge readout section is not broken.

The characteristics of the above-described TFT are determined by the potential difference (Vgs) between the gate and the source.
However, there is no structural difference between the drain and the source, and a lower potential of the two terminal voltages of the drain and the source is a physically significant source.

FIG. 13 shows a diode simulation circuit using this type of TFT and its characteristics. In FIG. 13A, when the drain D of the TFT 1 is connected to the gate G and the source S is connected to GND (ground), the current flowing through the TFT 1 changes according to the voltage of the drain D.

When the drain voltage Vd is positive, the source in the physical sense is the terminal of the source S because the drain voltage Vd is higher than the source voltage Vs. At this time, since Vgs = Vg−Vs = Vd−Vs ≧ 0,
As Vd increases, Vgs also increases, and the current-voltage characteristics also change. As shown in FIG. 13B, when Vgs exceeds the threshold voltage Vth, the current sharply increases.

On the other hand, when the drain voltage Vd is negative,
The source in the physical sense is a terminal of the drain D because the source voltage Vs is higher than the drain voltage Vd.
At this time, Vgs = Vg−Vd = 0, and Vgs is 0 even if Vd decreases. For this reason, since Vgs does not change, the current hardly changes. Therefore, this circuit exhibits a diode-like characteristic as shown in FIG.

[0019]

SUMMARY OF THE INVENTION However, TFT
It is difficult to control the threshold voltage Vth of 1 in manufacturing, and the threshold voltage cannot be set to an arbitrary threshold voltage distributed in a certain voltage region regardless of the size of the TFT or the like. Was.

That is, when the threshold voltage is lower than the normal operation region voltage (the maximum value of the voltage due to the charges in the charge storage section read out to the charge reading section as a pixel signal; also referred to as the maximum reading voltage), the normal operation is performed. A voltage breakdown occurs within the range of the region voltage, and it becomes impossible to extract an accurate pixel signal. Therefore, when the TFT is used as a protection circuit, it is necessary to set the threshold voltage to be equal to or higher than the normal operation area voltage.

Also, due to the characteristics of the TFT 1, the current value flowing from the drain to the source does not become the minimum value when Vgs = 0, but the current value becomes the minimum value when the predetermined voltage Vgs <0. have.

However, in the conventional method, V
Only the area of gs ≧ 0 is used. For this reason, TFT
Is used as a protection circuit, the leak current in the normal operating voltage region is not the minimum value, but will destroy the pixel signal. As a result, an accurate image cannot be read out, and it is necessary to sufficiently reduce the leak current.

An object of the present invention is to provide an X-ray imaging apparatus having a protection circuit capable of setting a threshold voltage to be equal to or higher than a normal operation region voltage and minimizing a leak current.

[0024]

The present invention employs the following means in order to solve the above-mentioned problems. The invention according to claim 1, wherein the charge conversion means is provided corresponding to a plurality of pixels arranged on the detection surface and converts incident X-rays into charges, and the charge conversion means is provided corresponding to the charge conversion means. Charge storage means for storing the charge converted by the conversion means; charge read means for reading the charge stored in the charge storage means; one end connected to the charge storage means; And a sweeping means for sweeping out the charge accumulated in the charge accumulating means when the above is obtained,
The gist is that the sweeping means is configured to be capable of changing the threshold voltage.

According to the present invention, the sweeping means sweeps out the charge stored in the charge storage means when the applied voltage becomes equal to or higher than the threshold voltage, but changes the threshold voltage. By shifting the characteristics of the pixel voltage and the leak current, the apparent threshold voltage can be increased,
Further, leakage current in the operation region can be reduced.

[0026] In claim 2, the sweeping means includes:
At least one field effect transistor, one of a drain and a source of the field effect transistor is connected to an output terminal of the charge storage means, and the other of a drain and a source of the field effect transistor is connected to a constant voltage source. Is the gist.

According to a third aspect of the present invention, the sweeping means is configured so that the threshold voltage can be changed by changing the voltage of the constant voltage source. The gist of the present invention is that the gate of the field effect transistor is connected to the charge storage means.

In the fifth aspect of the present invention, the gate of the field effect transistor is connected to the charge storage means via a potential difference generating means for generating a predetermined potential difference, and the gate is changed by changing the potential difference generated by the potential difference generating means. The gist is that the threshold voltage can be changed.

According to the present invention, the potential difference generating means for generating a predetermined potential difference is inserted between the gate of the field-effect transistor and the charge storage means, and the potential difference generated by the potential difference generating means is changed to change the threshold value. Since the voltage is changed, the apparent threshold voltage can be increased, and the leak current can be reduced.

According to a sixth aspect of the present invention, there is provided a storage means for storing correction data of a leak current flowing through the sweeping means, and a correction means for correcting image data read from the charge storage means based on the correction data. The gist is to provide.

According to this invention, the correction data of the leak current flowing through the sweeping means is stored in the storage means, and the correction means corrects the image data read from the charge storage means based on the correction data. Thus, image data in which the leakage current has been corrected can be obtained.

Accordingly, the leak current in the normal operation region can be reduced to the limit of the field effect transistor as required, and the deterioration of the pixel signal can be prevented.

According to a seventh aspect of the invention, the gist is that the correction data is obtained based on a signal read through the charge reading means when no X-rays are incident.

According to the present invention, a plurality of charge conversion means are provided corresponding to a plurality of pixels arranged on the detection surface and convert incident X-rays into charges, and a plurality of charge conversion means correspond to the charge conversion means. A charge storage means for storing the charge converted by the charge conversion means; a charge reading means for reading the charge stored in the charge storage means; and an electric field having one end connected to an output terminal of the charge storage means. An effect transistor is provided, and a sweeping means for sweeping out the charge stored in the charge storage means when a voltage at an output terminal of the charge storage means becomes equal to or higher than a predetermined voltage.

According to the present invention, the charge converting means converts the incident X-rays into charges, the charge storing means stores the charges converted by the charge converting means, and the charge reading means stores the charges in the charge storing means. The read out charge is read out, and the field effect transistor sweeps out the charge stored in the charge storage means when the voltage at the output terminal of the charge storage means becomes equal to or higher than a predetermined voltage. A pixel signal having a voltage lower than the voltage can be accurately extracted.

[0036]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Some embodiments of the X-ray imaging apparatus according to the present invention will be described below in detail with reference to the drawings.

<First Embodiment> First, the first embodiment of the X-ray imaging apparatus of the present invention will be described.

(Example 1) FIG. 1 shows a configuration diagram of Example 1 of the X-ray imaging apparatus according to Embodiment 1. The X-ray imaging apparatus according to the first embodiment illustrated in FIG. 1 is a pixel charge transfer type imaging apparatus, and includes an X-ray flat panel detector 2, a vertical shift register 15, a read amplifier 18, and a multiplexer 19.

A plurality of pixels are two-dimensionally arranged on a detection surface on the plane of the X-ray flat panel detector 2. Each pixel includes a photoelectric conversion unit 5 as a charge conversion unit, a charge storage unit 6 as a charge storage unit, a protection TFT 7 as a sweeping unit,
A TFT 14 is provided as charge reading means. Each TF
T is a thin film transistor, which is composed of a field effect transistor.

The photoelectric converter 5 converts incident light or incident X-rays into electric charges, and generates electric charges corresponding to the incident light or incident X-rays. The photoelectric conversion unit 5 is represented by a capacitor as shown in FIG. 1 as an electrical equivalent circuit. The storage capacitor unit 6 stores the charge generated in the photoelectric conversion unit 5,
The electrical equivalent circuit is represented by a capacitor as shown in FIG.

The photoelectric conversion unit 5 is, for example, selenium which directly converts X-rays into electric charges, but is not limited to selenium. When using this selenium, it is necessary to apply a voltage of several KV to both ends of the selenium film. In the case where strong X-rays are applied, for example, several KV may be applied to the storage capacitor 6. is there.

The transfer TFT 14 is connected to the charge storage section 6. The transfer TFT 14 is a readout SW that reads out the charge accumulated by the charge accumulation unit 6 to the readout amplifier 18 via the readout signal line 17.

After the end of the X-ray exposure, the transfer TFT 14
When a gate control signal is input to the gate G from the vertical shift register 15 via the vertical selection line 16, the drain D and the source S are turned on and operate as a switch.
After the end of the X-ray exposure, the electric charge having the X-ray image information accumulated in each pixel is read out to an external readout amplifier 18 via the transfer TFT 14.

Each vertical selection line 16 is connected to the gates G of the three transfer TFTs 14 in the row corresponding to its own vertical selection line. Each read amplifier 18 has a drain D of three transfer TFTs 14 in a column corresponding to its own read amplifier.
, And reads out the charges of the three transfer TFTs 14 in the corresponding column and outputs the charges to the multiplexer 19.

It is to be noted that an integration type amplifier in which a capacitor 18b is connected between the inverting input terminal and the output terminal of the read amplifier 18 is used. The multiplexer 19 converts a parallel output from each read amplifier 18 into a serial output,
The signal is sent to an analog / digital (A / D) converter (not shown).

Each protection TFT 7 constitutes a protection circuit for preventing the signal voltage due to the electric charge accumulated in the accumulation capacitance section 6 from abnormally increasing. The protection TFT 7 is directly connected (short-circuited) between the drain and the gate. One end of the source is connected to a voltage-changing terminal (the midpoint between the photoelectric conversion unit 5 and the storage capacitor unit 6) of the storage capacitor unit 6 of each pixel, and a predetermined potential is connected to the fixed potential terminal 10 at the other end of the protection TFT. Is set to a positive fixed potential.

In the example shown in FIG. 1, each protection TFT 7 is
The drain D, which is one end, is connected to the source S, which is the input side of the transfer TFT 14, and the drain and gate are directly connected (short-circuited). The fixed potential terminal 10 on the source S side has a positive fixed potential s. [V] is set.

Each of the protection TFTs 7 is an element having a non-linear resistance action, and when a voltage higher than a predetermined threshold value lower than a voltage at which the transfer TFT 14 is destroyed is applied, a sharp current flows, The charge supplied from the charge storage unit 6 to the source S of the transfer TFT 14 is swept out.

Next, the operation of the X-ray imaging apparatus according to the first embodiment of the present invention will be described. In this detector, the applied voltage is distributed to the capacity of the photoelectric conversion unit 5 and the capacity of the charge storage unit 6 during X-ray irradiation. After the end of the X-ray exposure, the electric charge having the X-ray image information stored in each storage capacitor unit 6 is read out to the readout amplifier 18 via the transfer TFT 14.

Here, since the threshold voltage of the protection TFT 7 is set lower than the breakdown voltage of the transfer TFT 14, even if a potential higher than the breakdown voltage is generated in the storage capacitor 6, the protection TFT 7 is completely protected. A current flows through the TFT 7. As a result, the transfer TF which is the read SW
T14 is no longer destroyed.

Here, when the gate G and the drain D of the protection TFT 7 are short-circuited and the source S is connected to GND, the drain voltage (pixel voltage) and the current have current characteristics as shown in FIG. The curve C1 results. The threshold voltage (breakdown voltage) is a voltage at which a current rapidly flows, and is a substantially constant voltage regardless of the size of the protection TFT 7 or the like.

Now, as pixel signals, 0 to x [V] of the pixel voltages shown in FIG. 2 are used as normal operation region voltages (x [V] is referred to as a maximum read voltage), and normal operation region voltages x When a signal of [V] or more is not required, the threshold voltage y [V] of the TFT 7 serving as the protection circuit needs to be higher than x [V].

However, when the source S of the TFT 7 serving as the protection circuit is connected to GND, the threshold voltage at that time becomes z [V] as shown by the current characteristic curve C1 in FIG.
And z [V] is often smaller than x [V].

Therefore, in the first embodiment, the protective TFT
The fixed potential terminal 10 of the source S 7 is set to s [V] as a positive fixed potential by a constant voltage source (not shown). As a result, the pixel voltage and the current become a current characteristic curve C2 as shown in FIG. 2, and the apparent threshold voltage becomes
y = z + s [V].

That is, by changing the voltage of the constant voltage source so that s> x−z, the voltage on the source side of the protection TFT 7 is set, and the threshold voltage is changed. When the voltage applied to the storage capacitor section 6 becomes equal to or higher than the normal operation area voltage x [V], the TFT 7 serving as a protection circuit
Can be turned on.

That is, since the protective TFT 7 does not cause a voltage breakdown within the normal operation area voltage x [V], an accurate pixel signal can be obtained.

At the same time as increasing the apparent threshold voltage y, as can be seen from FIG. 2, the leakage current in the normal operation region is also reduced from the current when Vgs = several volts by V
It can be reduced to the current when gs = 0.

When the voltage of the storage capacitor 6 drops abnormally, it is necessary to reverse the connection between the drain and the source of the protection TFT 7.

As described above, by increasing the voltage on the source side of the protection TFT 7, the characteristics of the pixel voltage and the leakage current can be shifted, and the apparent threshold voltage can be increased. Can be reduced.

Therefore, a signal required as a pixel signal can be read more accurately, the pixel signal does not abnormally rise above the withstand voltage of the TFT 7, and the TFT is prevented from being destroyed.

(Embodiment 2) FIG. 3 shows a configuration diagram of Embodiment 2 of the X-ray imaging apparatus according to Embodiment 1. The X-ray imaging apparatus according to the second embodiment illustrated in FIG. 3 is an imaging apparatus of a pixel signal amplification type, and includes an X-ray flat panel detector 3, a vertical shift register 15, and a multiplexer 19.

Each pixel on the plane of the X-ray flat panel detector 3 includes a photoelectric conversion unit 5, a charge storage unit 6, a protection TFT 7, and an amplification T
FT22, vertical selection TFT23, reset TFT2
4 is provided.

Each protection TFT 7 constitutes a protection circuit for preventing the signal voltage due to the electric charge accumulated in the accumulation capacitor section 6 from abnormally increasing. The protection TFT 7 is directly connected (short-circuited) between the drain and the gate, and the drain is connected to the drain. One end of the source is connected to a voltage-variable terminal of the storage capacitor 6 of each pixel, and a protective TF is connected.
S [V] is set as a fixed potential at a fixed potential terminal 10 of the source of T7 by a constant voltage source.

One end of the charge storage section 6 is connected to the gate G of the amplifying TFT 22 for amplifying the charge stored in the charge storage section 6, and the source S of the amplifying TFT 22 has a read signal for reading the stored charge. The line 17 is connected, and the vertical selection TFT 23 is connected to the drain D of the amplification TFT 22.
Are connected.

The gate G of the load TFT 25 is connected to the gate line 26, the source S of the load TFT 25 is connected to the source line 27, and the drain D of the load TFT 25 is connected to the read signal line 17. TFT2 for each load
5 is three amplifying TFTs corresponding to three lines (three rows)
22 are connected to the source S. Vg of load TFT25
s and Vgs of the amplifying TFT 22 have the same value, and a current flows from the load TFT 25 to the amplifying TFT 22.

The gate G of the three vertical selection TFTs 23 for each row is connected to the vertical selection line 1 from the vertical shift register 15.
6 is connected. After the X-ray irradiation is completed, the vertical control TFT 23 receives the gate control signal from the vertical shift register 15 to the gate G, and thereby the drain D / source S
The switch turns on and operates as a switch, and the load TFT 25
A current is supplied to the TFT 22 for amplification.

When the three vertical selection TFTs 23 in one line are turned on, each of the three amplification TFTs 22 in one line reads the charge of the TFT 7 and outputs it to the multiplexer 19 via the read signal line 17.

A reset line 21 from the vertical shift register 15 is connected to the gate G of the reset TFT 24,
The drain S of the protection TFT 7 is connected to the source S of the reset TFT 24, and the reset line 28 is connected to the drain D of the reset TFT 24. T for each reset
The FT 24 resets the charge of the protection TFT 7 for each row.

The multiplexer 19 converts the parallel output from the three amplifying TFTs 22 for each row into a serial output, and sends it to an analog / digital (A / D) converter (not shown).

According to the X-ray imaging apparatus of Embodiment 2 configured as described above, the fixed potential terminal 10 of the source S of the protection TFT 7 is set to s as a positive fixed potential by a constant voltage source.
[V] is set. As a result, the pixel voltage and the current become a current characteristic curve C2 as shown in FIG. 2, and the apparent threshold voltage becomes y = z + s [V].

That is, by changing the voltage of the constant voltage source so that s> x−z, the voltage on the source side of the protection TFT 7 is set, and the threshold voltage is changed. When the voltage applied to the storage capacitor section 6 becomes equal to or higher than the normal operation area voltage x [V], the TFT 7 serving as a protection circuit
Can be turned on.

That is, since the protective TFT 7 does not cause a voltage breakdown within the normal operation area voltage x [V], an accurate pixel signal can be extracted.

At the same time as increasing the apparent threshold voltage y, as can be seen from FIG. 2, the leakage current in the normal operation region is also reduced from the current when Vgs = several volts by V
It can be reduced to the current when gs = 0.

If the voltage of the storage capacitor 6 drops abnormally, it is necessary to reverse the connection between the drain and source of the protection TFT 7. Further, as the signal readout method, the pixel charge transfer type imaging device shown in FIG. 1 and the pixel signal amplification type imaging device shown in FIG. 3 have been described, but the present invention is not limited to these devices.

Further, in the first embodiment, the protective TFT
Although the drain and gate of 7 are short-circuited, for example, a Zener diode may be used instead of the protection TFT 7. In this case, if the anode of the Zener diode is connected to the voltage-changing terminal of the storage capacitor unit 6 and the cathode is set to s [V] as a fixed potential, the same effect as that of the first embodiment can be obtained.

<Second Embodiment> Next, a second embodiment of the X-ray imaging apparatus of the present invention will be described. FIG. 4 shows a configuration diagram of the protection circuit according to the second embodiment. The protection circuit 13a shown in FIG.
Are provided for each pixel, and a plurality of protective TFTs for preventing the signal voltage of the storage capacitor 6 from abnormally increasing.
7-1 and 7-n.

In each of the plurality of protection TFTs 7-1 and 7-n, the drain D and the gate G are short-circuited and connected in series. The drain D, which is one end of the protection TFT 7-1, is connected to the voltage-changing terminal of the storage capacitor 6, and the source S, which is the other end of the protection TFT 7-n, is set to s [V] as a fixed potential. I have. The configuration of the photoelectric conversion unit 5 is the same as that shown in the first embodiment.

The other configuration of the second embodiment is the same as the configuration of the first embodiment, and a detailed description thereof will be omitted.

The signal readout method may be the pixel charge transfer type X-ray imaging device of the first embodiment shown in FIG. 1 or the pixel signal amplification type X-ray imaging device of the second embodiment shown in FIG. Alternatively, the present invention is not limited to these X-ray imaging devices.

Next, in the X-ray imaging apparatus thus configured, a plurality of protective T
The operation of the FTs 7-1 and 7-n will be described.

First, when the gate G and the drain D of the protection TFT 7 are short-circuited and the source S is connected to GND, the drain voltage and the current become current characteristic curves C as shown in FIG.
1 (the same characteristic as the current characteristic curve C1 in FIG. 2). When the source S of one protection TFT 7 is connected to GND, the threshold voltage is z [V], and z [V] is x
Often smaller than [V].

If the protection TFTs 7 in which the gate G and the drain D are short-circuited are a plurality of protection TFTs 7-1 and 7-n connected in series, the threshold voltage at that time is as follows. It is about n times the threshold voltage when there is one TFT 7 for use. That is, the drain voltage and the current become a current characteristic curve C3 as shown in FIG. 5, and the apparent threshold voltage becomes y = nz [V].

Therefore, when the number of protection TFTs 7 to be connected in series is set so that n> x / z, the voltage applied to the storage capacitor 6 becomes equal to or higher than the normal operation region voltage x [V]. Then, the protection TFTs 7-1 to 7-n can be turned on.

That is, since the protective TFTs 7-1 to 7-n do not break down within the normal operation area voltage x [V],
An accurate pixel signal can be extracted.

When the voltage of the storage capacitor unit 6 drops abnormally, the drain TFTs of the protection TFTs 7-1 and 7-n are not connected.
The source connection needs to be reversed.

As described above, the threshold voltage can be increased by connecting a plurality of the protection TFTs 7.

In the first embodiment, the protection TFT 7
However, for example, a plurality of serially connected Zener diodes may be used instead of the plurality of protective TFTs 7 connected in series.
In this case, if the anode of the first Zener diode is connected to the voltage-variable terminal of the storage capacitor section 6 and the cathode of the last Zener diode is set to s [V] as a fixed potential, the effect of the second embodiment can be obtained. Similar effects can be obtained.

Third Embodiment Next, a third embodiment of the X-ray imaging apparatus according to the present invention will be described. First, due to the characteristics of the TFT, the current value becomes the minimum value not when Vgs = 0, but has the minimum value at a predetermined voltage at Vgs <0. However, the conventional method uses only the region of Vgs ≧ 0.

Therefore, in the third embodiment, in order to reduce the leak current, the structure which always operates while satisfying Vg-Vd = constant <0 is adopted. That is, the protection circuit according to the third embodiment utilizes the region of Vgs <0.

FIG. 6 shows a configuration diagram of the protection circuit according to the third embodiment. The protection circuit 13b shown in FIG. 6 includes a protection TFT 7 for preventing the signal voltage of the storage capacitor unit 6 from abnormally increasing.
And a control TFT 11 and a transforming TFT 12, which are provided for each pixel.

The drain D of the control TFT 11 is connected to the T
The source S of the FT 12 is connected, and the drain D of the protection TFT 7 is connected to the gate G of the transformation TFT 12. Further, the gate G of the protection TFT 7 is connected to the source S of the transformation TFT 12.

One end (for example, drain D) of the drain and source of the protection TFT 7 is connected to a voltage-varying terminal of the storage capacitor section 6 of each pixel, and the fixed potential of the other end (for example, source S) of the protection TFT 7 is connected. The terminal 10 is set to a predetermined fixed potential by a constant voltage source (not shown).

Control TFT 11 and Transformation TFT 12
Are connected in series to constitute a source-follower circuit and constitute a voltage difference generating means, and generate a predetermined potential difference between the drain and the gate of the protection TFT 7. Constant potential (Vg
−Vd = constant <0). The threshold voltage can be changed by changing the potential difference generated by the voltage difference generating means. Hereinafter, the configurations of the control TFT 11 and the transforming TFT 12 will be described.

The current flowing through the TFTs 11 and 12 is the voltage V
It is determined by the gate-source voltage Vgs11 of the control TFT 11 without depending on ds. Now, the current flows from the source S of the transformation TFT 12 to the gate G of the protection TFT 7.
In the case where it hardly flows into the control TFT 11,
Has the same value as the current flowing through the transforming TFT 12.

According to the above-mentioned characteristics, in order for the current flowing through the control TFT 11 to be the same as the current flowing through the transforming TFT 12, Vgs of the control TFT 11 must be equal to the transforming TFT.
It must be the same value as 12 Vgs.

Therefore, even if Vg12 of the transforming TFT 12 changes, Vgs11 = Vgs12 = Vg12−Vs1.
Vs12 is determined so that the relationship of 2 holds.

By providing such a circuit between the drain and gate of the protection TFT 7, V (7-G) -V
(7-D) = V (12-S) -V (12-G) =-Vg
The relationship of s11 = −d <0 holds. That is, between the drain and the gate of the protection TFT 7, Vgd <
It becomes 0.

The means for providing a potential between the drain and the gate of the protection TFT 7 is not limited to the two control TFTs 11 and the transforming TFT 12.

As a signal reading method, a pixel charge transfer type X-ray imaging device shown in FIG. 1 or a pixel signal amplification type X-ray imaging device shown in FIG. 3 may be used. The signal reading method is not limited to these methods.

Next, the operation of the protection circuit 13b will be described.
First, when the gate G and the drain D of the protection TFT 7 are short-circuited and the source S is connected to GND, the drain voltage and the current become the current characteristic curve C1 shown in FIG.

Now, when 0 to x [V] is used as a pixel signal and a signal of x [V] or more is not required, the threshold voltage y [V] of the protection circuit 13b is x [V]. Needed to be bigger.

If the voltage between the gate and source of the control TFT 11 is d [V], the apparent threshold voltage of the TFT 7 for protection is y = z + d as shown by the current characteristic curve C4 in FIG. [V].

For this reason, by setting the voltage between the gate and the source of the control TFT 11 so that d = x−z, the voltage applied to the storage capacitor 6 becomes equal to or higher than the normal operation region voltage x. Then, the protection circuit 13b can be operated.

When V (7-D) is the drain voltage of the protection TFT 7 and V (7-S) is the source voltage of the protection TFT 7, Vgs of the protection TFT 7 is determined as follows. You.

(1) When V (7-D)> V (7-S), the source in the physical sense is the source terminal 7-S, and Vgs = V (7-D) -d-V (7-S)>-d (2) When V (7-D) <V (7-S), the source in the physical sense is the drain terminal 7-D, and Vgs = -d.

That is, even if V (7-D)> V (7-S), if V (7-D) -V (7-S) <d,
Vgs <0, and the leak current can be sufficiently reduced.

Further, by setting the fixed potential of the source S of the protection TFT 7 to s [V], the apparent threshold voltage becomes y = z + s + d [V] as shown in FIG.

At the same time as increasing the apparent threshold voltage, as shown in FIG. 8, the leakage current in the normal operation region is reduced from the current when Vgs = about several V to Vgs = −
The current can be reduced to d [V].

When the voltage of the storage capacitor section 6 drops abnormally, the connection between the drain and the source of the protection TFT 7 needs to be reversed.

As described above, in the third embodiment, the protection T
By inserting a circuit whose gate voltage is always lower than the drain voltage of the FT 7 between the drain and the gate of the protection TFT 7, the apparent threshold voltage can be increased arbitrarily. Since the negative region can be used, the leak current can be further reduced.

Therefore, a signal required as a pixel signal can be read more accurately, the pixel voltage can be prevented from abnormally increasing beyond the withstand voltage of the TFT, and destruction of the TFT can be avoided.

<Fourth Embodiment> Next, a fourth embodiment of the X-ray imaging apparatus of the present invention will be described below. FIG. 9 shows a configuration diagram of an X-ray imaging apparatus according to the fourth embodiment.

The X-ray imaging apparatus shown in FIG. 9 includes a vertical shift register 15, an imaging panel 32, a multiplexer 19, an analog / digital converter (A / D) 33, a correction data image memory 30, an image processing device 29, and a display. Device 31
a, storage device 30b.

The imaging panel 32 includes the X-ray flat panel detector 2 and the readout amplifier 18 shown in FIG. The multiplexer 19 converts a parallel output from the imaging panel 32 into a serial output, and an analog / digital converter (A / D) 33 converts a signal from the multiplexer 19 into a digital signal.

The correction data image memory 30 stores the A / D3
The correction data of the leakage current flowing from the protection TFT 7 flowing from the protection TFT 7 is collected and stored. This correction data is stored in the charge reading means (for example, FIG. 1) when no light or X-rays are incident.
Is obtained based on a signal read through the transfer TFT 14) shown in FIG.

The image processing device 29 constitutes a correction means,
Image data is collected from the A / D 33 by a second current obtained by adding the leak current to the first current due to the charge from the storage capacitor unit 6 when light or X-rays are incident, and the collected image data and correction data are collected. By obtaining difference image data from the correction data stored in the image memory 30 for use, image data based on the first current whose leak current has been corrected is obtained.

The display device 31a displays the image data in which the leak current from the image processing device 29 has been corrected. The storage device 30b stores the image data from which the leak current from the image processing device 29 has been corrected.

In such a configuration, the protection TFT 7
A region where the pixel voltage (the voltage of the storage capacitor unit 6 shown in FIG. 1) is lower than the source voltage (the source S shown in FIG. 1) is set as a normal operation region. In this normal operation region, the leak current is substantially constant even if the pixel value changes. For this reason,
The leak current at the time of reading one pixel is almost constant regardless of the pixel signal.

Therefore, by collecting images when no X-rays are incident, it is possible to measure the leak current by the protection circuit of each pixel. Then, the correction data based on the leak current is stored in the correction data image memory 30.

Next, an image actually captured by X-ray incidence is input to the image processing device 29. The image processing device 29
Difference image data between the input image data and the correction data stored in the correction data image memory 30 is obtained. As a result, image data in which the leak current has been corrected can be obtained.

Therefore, the leak current in the normal operation region can be reduced to the limit of the TFT as required, and thus the deterioration of the pixel signal can be prevented.

<Fifth Embodiment> Next, an X-ray imaging apparatus according to a fifth embodiment of the present invention will be described. FIG. 10 shows a fifth embodiment.
1 is a configuration diagram of a protection circuit of the X-ray imaging apparatus of FIG. In this X-ray imaging apparatus, a plurality of pixels are arranged on a plane, and each pixel generates a charge according to incident light, a photoelectric conversion unit 5, a storage capacitor unit 6 for storing the generated charge, and a storage capacitor unit. 6 is provided with a plurality of protection TFTs 7-1 and 7-n for preventing the signal voltage from abnormally increasing.

The plurality of protection TFTs 7-1 and 7-n are connected in series, and one of the two ends thereof is one of the protection TFTs.
A short circuit between the drain of 7-1 and all the gates is provided for each pixel. The drain D, which is one end of the protection TFT 7-1, is connected to the voltage-changing terminal of the storage capacitor 6, and the source S, which is the other end of the protection TFT 7-n, is set to s [V] as a fixed potential. I have.

As the photoelectric conversion unit 5, there is selenium which can directly convert X-rays into electric charges. When using this selenium, it is necessary to apply a voltage of several KV to both ends of the selenium film.
When a line is irradiated, there is a possibility that several KV may be applied to the storage capacitor part. However, as the photoelectric conversion unit 5,
It is not limited to selenium.

The signal readout method may be the pixel charge transfer type X-ray imaging device of the first embodiment shown in FIG. 1 or the pixel signal amplification type X-ray imaging device of the second embodiment shown in FIG. Alternatively, the present invention is not limited to these X-ray imaging devices.

Next, in the X-ray imaging apparatus thus configured, a plurality of protective T
The operation of the FTs 7-1 and 7-n will be described.

First, when the gate G and the drain D of the protection TFT 7 are short-circuited and the source S is connected to GND, the drain voltage and the current have a current characteristic curve C1 as shown in FIG.

The voltage at which the current suddenly flows is called the threshold voltage, and is substantially constant regardless of the size of the TFT 7 or the like.

Now, a plurality of protection TFTs 7-1 to 7-n
Are connected in series, each gate is shared and connected to the drain of the protection TFT 7-1 which is one end, so that the threshold voltage is almost the same as in the case where only one protection TFT 7 is used. Although it does not change, the current characteristic curve C6 in FIG.
As shown in (1), the leakage current in the normal operation range can be reduced.

<Sixth Embodiment> Next, an X-ray imaging apparatus according to a sixth embodiment of the present invention will be described. FIG. 12 shows a sixth embodiment.
1 is a configuration diagram of a protection circuit of the X-ray imaging apparatus of FIG. The protection circuit 13c shown in FIG. 12 includes a protection TFT 7 for preventing the signal voltage of the storage capacitor unit 6 from abnormally increasing, a transformation capacitor 34, and a transformation charge supply TFT 35,
It is provided for each pixel.

A transforming capacitor 34 is connected between the gate and the drain of the protective TFT 7, and the transforming charge supplying TFT 3 is connected.
5 is connected to the gate of the protection TFT 7.

With no charge in the storage capacitor section 6 (before the signal is stored or immediately after the signal is read), the gate of the transforming charge supply TFT 35 is turned on, and the charge is supplied to the transforming capacitor 34. At this time, the protection TFT
7 is 0 [V], so that the variable charge supply T
The voltage −d [V] applied to the source of the FT 35 is applied to both ends of the transforming capacitor 34.

Here, the transforming charge supply TFT 35 is turned off.
Set to F state and start shooting. At this time, since the electric charge stored in the transforming capacitor 34 does not change, the voltage between both ends of the transforming capacitor 34 does not change. That is, regardless of the voltage of the drain of the protection TFT 7, the protection TFT 7
7, the potential difference between the gate and the drain is kept at -d [V]. Subsequent processing is the same as that of the third embodiment, and a description thereof will not be repeated.

Note that the present invention can be implemented with any of the indirect conversion type and the direct conversion type flat detector.

[0135]

According to the present invention, the sweeping means sweeps out the charge accumulated in the charge accumulating means when the applied voltage becomes equal to or higher than the threshold voltage, but changes the threshold voltage. Therefore, the characteristics of the pixel voltage and the leak current can be shifted, the apparent threshold voltage can be increased, and the leak current in the operation region can be reduced.

Further, since a potential difference generating means for generating a predetermined potential difference is inserted between the gate of the field effect transistor and the charge storage means, and the potential difference generated by the potential difference generating means is changed, the threshold voltage is changed. The apparent threshold voltage can be increased, and the leak current can be reduced.

Further, the correction data of the leak current flowing through the sweeping means is stored in the storage means, and the correction means corrects the image data read from the charge accumulating means based on the correction data. The obtained image data is obtained. Therefore, the leakage current in the normal operation region can be reduced to the limit of the field effect transistor as needed, and the deterioration of the pixel signal can be prevented.

The charge conversion means converts the incident X-rays into charges, and the charge storage means stores the converted charges.
The charge readout unit reads out the stored charge, and the field-effect transistor sweeps out the charge stored in the charge storage unit when the voltage at the output terminal of the charge storage unit exceeds a predetermined voltage. Can be accurately read out without destroying an image signal having a voltage less than the predetermined voltage.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a pixel charge transfer type X-ray imaging device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a current characteristic curve of the protection circuit according to the first embodiment.

FIG. 3 is a configuration diagram of a pixel signal amplification type X-ray imaging apparatus according to Embodiment 1 of the present invention.

FIG. 4 is a configuration diagram illustrating a protection circuit according to a second embodiment.

FIG. 5 is a diagram showing a current characteristic curve of the protection circuit according to the second embodiment.

FIG. 6 is a configuration diagram illustrating a protection circuit according to a third embodiment.

FIG. 7 is a diagram illustrating a current characteristic curve 1 of the protection circuit according to the third embodiment;

FIG. 8 is a diagram illustrating a current characteristic curve 2 of the protection circuit according to the third embodiment;

FIG. 9 is a configuration diagram of an X-ray imaging apparatus according to Embodiment 4 of the present invention.

FIG. 10 is a configuration diagram illustrating a protection circuit according to a fifth embodiment.

FIG. 11 is a diagram showing a current characteristic curve of the protection circuit according to the fifth embodiment.

FIG. 12 is a configuration diagram illustrating a protection circuit according to a sixth embodiment.

FIG. 13 is a diagram showing a diode simulation circuit using a conventional thin film transistor and its characteristics.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 TFT 5 Photoelectric conversion part 6 Storage capacity part 7 Protection TFT 8 Signal output terminal 10 Fixed potential terminal 11 Control TFT 12 Transformation TFT 13a, 13b Protection circuit 14 Transfer TFT 15 Vertical shift register 16 Vertical selection line 17 Read signal Line 18 Integrator 19 Multiplexer 20 Image signal output terminal 21, 28 Reset line 22 Amplification TFT 23 Vertical selection TFT 24 Reset TFT 25 Load TFT 26 Load TFT gate line 27 Load TFT source line 29 Image processing Device 30 Image memory for correction data 31a Display device 31b Recording device 32 Imaging panel 33 A / D

Claims (8)

[Claims] A charge conversion unit provided to correspond to a plurality of pixels arranged on a detection surface and converting incident X-rays into charges; and a charge conversion unit provided to correspond to the charge conversion unit. Charge accumulating means for accumulating the electric charge converted by the charge accumulating means; charge reading means for reading the electric charge accumulated in the charge accumulating means; one end connected to the charge accumulating means; An X-ray imaging apparatus, comprising: a sweeping means for sweeping out the electric charge accumulated in the electric charge accumulating means when the electric charge is accumulated, wherein the sweeping means is configured to change the threshold voltage. 2. The sweeping means includes at least one field effect transistor, one of a drain and a source of the field effect transistor is connected to an output terminal of the charge storage means, and a drain or a source of the field effect transistor is 2. The X-ray imaging apparatus according to claim 1, wherein the other is connected to a constant voltage source. 3. The apparatus according to claim 2, wherein said sweeping means is configured to change said threshold voltage by changing a voltage of said constant voltage source.
Line imaging device. 4. A gate of the field effect transistor,
The X-ray imaging apparatus according to claim 2, wherein the X-ray imaging apparatus is connected to the charge storage unit. 5. A gate of the field effect transistor,
3. The apparatus according to claim 2, wherein said threshold voltage is changeable by changing a potential difference generated by said potential difference generating means and connected to said charge storage means via a potential difference generating means for generating a predetermined potential difference. The X-ray imaging apparatus according to claim 1. 6. A storage unit for storing correction data of a leak current flowing through the sweeping unit, and a correction unit for correcting image data read from the charge storage unit based on the correction data. The X-ray imaging apparatus according to any one of claims 1 to 5, characterized in that: 7. The X-ray imaging apparatus according to claim 6, wherein said correction data is obtained based on a signal read through said charge reading means when X-rays are not incident. apparatus. 8. A plurality of charge conversion means provided corresponding to a plurality of pixels arranged on the detection surface and converting incident X-rays into charges, and a plurality of charge conversion means provided corresponding to the charge conversion means, Charge storage means for storing the charge converted by the conversion means, charge read means for reading the charge stored in the charge storage means, and a field effect transistor having one end connected to the output end of the charge storage means, An X-ray imaging apparatus, comprising: a sweeping means for sweeping out the charge stored in the charge storage means when a voltage at an output terminal of the charge storage means becomes equal to or higher than a predetermined voltage.