JP2000111651A - Radioactive ray two-dimensional detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To cut off an application of bias voltage on a bias electrode of a semiconductor layer by incidence of a large quantity of X-rays exceeding either prescribed value of a detecting element (a pixel). SOLUTION: A semiconductor layer 1 for converting X-rays into electric charge by sensing the X-rays and a switching element matrix 4 are integrally formed, and a common bias electrode 2 to a signal electrode 3 of the semiconductor layer 1 is connected to a bias power source E through a switch 20. An electric current flowing to the bias electrode 2 from the bias power source E is detected by an electric current detector 21 to be supplied to a control circuit 22. The control circuit 22 compares a detecting electric current value supplied from the electric current detector 21 with a threshold value to turn off the switch 20 when the detecting electric current value reaches the threshold value to cut off an impression of bias voltage on the bias electrode 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-dimensional radiation detector for detecting radiation such as X-rays suitable for medical diagnosis of an X-ray imaging apparatus, and more particularly to a solid-state radiation type two-dimensional detector.

[0002]

2. Description of the Related Art Radiation two-dimensional detectors of this type of solid-state operation system are disclosed in Japanese Patent Application Laid-Open Nos. 4-212456 and 4-2.
As disclosed in Japanese Patent Application Laid-Open No. 12458 and Japanese Patent Application Laid-Open No. Hei 3-185863, the semiconductor device includes a semiconductor layer that senses radiation such as X-rays or light converted from radiation and converts it into electric charges, and a field effect transistor. The switching element matrix is integrated with the switching element matrix, and the switching elements are two-dimensionally scanned to obtain an image signal. The configuration is shown in FIGS.

In FIG. 6, reference numeral 1 denotes a semiconductor layer, which is formed in a matrix in which a bias electrode 2 connected to a bias power source E is formed on one surface and detection elements (pixels) are formed on the other surface so as to be arranged vertically and horizontally. The signal electrode 3 is formed. Reference numeral 4 denotes a switching element matrix composed of switching elements 5 such as thin film transistors (TFTs). Each switching element 5 is connected to each signal electrode 3 of the semiconductor layer 1, and the semiconductor layer 1 and the switching element matrix 4 Manufactured by thin film technology. In the figure, reference numeral 6 denotes a capacitor for accumulating electric charges, which is manufactured by a thin-film technique like the semiconductor layer 1 and the switching element matrix 5.

In this configuration, radiation transmitted through the subject, for example, X-rays, passes through the bias electrode 2 and passes through the semiconductor layer 1.
X-rays are absorbed by the semiconductor layer 1 to generate electron-hole pairs (charges). The generated charges undergo a charge shift due to a voltage applied to the bias electrode 2 from the bias power supply E, and the charges are accumulated in the capacitor 6. The amount of the charges accumulated in the capacitor 6 is incident on the semiconductor layer 1. It depends on the X-ray dose to be performed.

The switching lines 7 of the switching elements 5 constituting the switching element matrix 4 are shown in FIG.
As shown in the equivalent circuit of FIG.
The readout line 9 is connected to a multiplexer 11 via an amplifier 10. When the switching element 5 is driven by the switching element drive circuit 8, the readout line 9 is connected to the capacitor 6 of each pixel of one line where the switching element 5 is turned on. The accumulated charges are simultaneously output to the readout line 9, and the switching element driving circuit 8 sequentially drives the switching elements 5, whereby the pixels are two-dimensionally scanned, and the signal output to the readout line 9 is output to the multiplexer 11. Is converted into an image signal for each pixel and input to the A / D converter 12, where A
A digital image signal for each pixel is obtained from the / D converter 12, and a two-dimensional X-ray image is obtained by processing the digital image signal.

[0006]

However, the above-mentioned conventional two-dimensional radiation detector has the following problems. In other words, when a large amount of radiation such as X-rays is incident on the semiconductor layer, the voltage applied to the capacitor increases due to the charging of the stored charge, and the increase in the voltage applied to the capacitor limits the dynamic range due to saturation of the stored amount. , And
This causes breakdown voltage breakdown of the switching element connected to the capacitor.

In view of the above circumstances, the present invention shuts off a bias voltage to a bias electrode of a semiconductor layer when a large amount of radiation X exceeding a predetermined value is incident on any one of the detection elements (pixels). The electron-hole pairs (charges) generated in the step are limited to eliminate the limitation of the dynamic range due to the saturation of the storage amount of the capacitor, and the switching element causes the withstand voltage breakdown by the rise of the voltage applied to the capacitor. It is an object of the present invention to provide a radiation two-dimensional detector which does not occur and provides a radiation two-dimensional detector having a wide dynamic range in which an image signal having a quantitative property corresponding to an incident X-ray dose can be obtained.

[0008]

In order to achieve the above object, in the radiation two-dimensional detector according to the present invention, a bias electrode of a semiconductor layer is connected to a bias power supply via a switch and the switch is connected to a bias power supply. And a control circuit for turning off the switch when the current value detected by the detecting means reaches a threshold value.

The radiation two-dimensional detector according to claim 2 is the radiation two-dimensional detector according to claim 1, wherein the radiation irradiation start time t0 and the radiation irradiation end time t
1, and a correction means for correcting the output signal q of each detection element (pixel) at the switch-off time t2 from the time t2 when the switch is turned off by the control circuit according to the following equation to obtain a correction signal Q: It is characterized by that. Q = (t1-t0) / (t2-t0) * q where q is an output signal of each pixel at the switch-off time t2.

With this configuration, in the radiation two-dimensional detector according to the first aspect, radiation, for example,
When X-rays enter, a current flows from a bias power supply to a bias electrode of a semiconductor layer. The current flowing through the bias electrode is
The current is detected by the current detecting means and supplied to the control circuit. The control circuit compares the current value detected by the current detection means with a predetermined threshold value, and when the detected current value exceeds the threshold value, activates a switch interposed between the bias electrode of the semiconductor layer and the bias power supply. Turn off. Note that the threshold value does not cause the saturation of the storage amount of the capacitor due to the rise in the voltage applied to the
In addition, a current value is set so that the switching element connected to the capacitor does not cause withstand voltage breakdown.

Accordingly, a large amount of X-rays is incident on one of the detection elements (pixels) of the semiconductor layer constituting the radiation two-dimensional detector, and a current flows from the bias power supply to the bias electrode of the semiconductor layer via the switch. When the threshold value is reached, the switch interposed between the bias electrode and the bias power supply is turned off and the application of the bias voltage to the bias electrode is cut off, suppressing the rise in the capacitor voltage and preventing the saturation of the accumulated amount. In addition to this, it is possible to prevent breakdown voltage of the switching element connected to the capacitor.

Further, in the radiation two-dimensional detector according to the second aspect, a large amount of X-rays are incident on one of the detection elements (pixels) of the semiconductor layer constituting the radiation two-dimensional detector,
The amount of electric charge accumulated in the capacitor from the X-ray irradiation start time t0 to the switch off time t2 when the switch interposed between the bias electrode and the bias power supply is turned off and the application of the bias voltage to the bias electrode is cut off X-ray irradiation start time t based on the output signal of each detection element (pixel)
Since the electric charge is corrected from 0 to the X-ray irradiation end time t1, an image signal having a quantitative property corresponding to the incident X-ray dose can be obtained, and the saturation of the accumulation amount of the capacitor and
Further, a radiation two-dimensional detector having a wide dynamic range without causing breakdown voltage of a switching element connected to a capacitor can be obtained.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing the configuration of a two-dimensional radiation detector according to one embodiment of the present invention. Components having the same functions as those in FIG. 6 are denoted by the same reference numerals, and description thereof will be omitted. In FIG. 1, reference numeral 20 denotes a switch, and 21 denotes a current detector, which is connected between the bias power source E and the bias electrode 2 of the semiconductor layer 1. 22 is a control circuit, and a capacitor 6
And a threshold value set to a current value at which the switching element 5 connected to the capacitor 6 does not cause withstand voltage destruction due to the rise of the voltage applied to the capacitor and the detection by the current detector 21. The switch 20 is turned off when the detected current value reaches a threshold value.

The signal electrodes 3 need to be formed in a matrix so that the detection elements (pixels) are arranged vertically and horizontally. The bias electrodes 2 are formed of a semiconductor layer as shown in FIG. Even if it is formed on the entire surface and is a single electrode common to all signal electrodes (not shown), it is also possible to use a single electrode as shown in FIGS.
As shown in (d), the electrode may be divided into a plurality of parts, and the divided electrodes may be common to only the signal electrodes of the group covered by the bias electrodes. In FIG. 2, (b) is an example in which three divided bias electrodes are vertically divided into three, and (c) is an example in which nine divided bias electrodes are vertically and horizontally divided, and (d) ) Shows an example in which a central part and a peripheral part thereof are divided into two divided bias electrodes. When the bias electrode is divided into a plurality of divided bias electrodes, each divided bias electrode is connected to the switch 20 and the current detector as shown in FIG.
A control circuit 22 is connected to the bias power source E via the series circuit 21 and turns off the switch 20 when the detected current value reaches a threshold value.

Next, the operation of the radiation two-dimensional detector having the configuration shown in FIG. 1 will be described. In FIG. 1, radiation from the top,
For example, when X-rays enter, the incident X-rays
To generate an electron-hole pair (charge) in the semiconductor layer 1, and the generated charge is applied to the bias electrode 2 of the semiconductor layer 1.
Causes a charge shift due to an electric field generated by the voltage applied to the capacitor 6, and this charge is accumulated in the capacitor 6.
Voltage rises.

The charge signal stored in the capacitor 6 is read out by sequentially driving the switching elements 5 of the switching element matrix 4 so that the charges stored in the capacitors 6 of the respective detection elements (pixels) for one line are respectively read. The signal is read out to the readout line, input to the multiplexer through the amplifier 10, and converted into a digital video signal for each pixel by the A / D converter. The above operation is
This is as described in the section of the prior art.

As shown in the drawing, a positive voltage is applied to the semiconductor layer, ie, a bias electrode, and a negative voltage is applied to the bias electrode. If it is small, the generated electrons and holes recombine before reaching the electrodes and disappear, and are not accumulated in the capacitor as electric charges. It is necessary to set a voltage that can be obtained from a product of the mobility and the lifetime and that can provide a sufficient electric field that does not recombine and disappear before reaching the electrode.

Returning to FIG. 1, the semiconductor layer 1 is irradiated with X-rays.
When an electron-hole pair (charge) is generated in the inside, the holes are attracted to the signal electrode 3 on the cathode side and charges are accumulated in the capacitor 6 connected thereto, while the electrons are transferred to the bias electrode 2 on the anode side. The current is drawn and a current flows from the bias power source E through the switch 20 which is normally on. This current is detected by the current detector 21, and the detected current value (detected current value signal) is given to the control circuit 22, and the control circuit 22 The detected current value signal is compared with the threshold value.

When a large amount of X-rays is incident on any of the detection elements (pixels) of the semiconductor layer, a large amount of electron-hole pairs (charges) are generated, and the current of the circuit including the switch 20 also increases.
When the detected current value signal reaches the threshold value, the control circuit 22 turns off the switch 20 and cuts off the application of the bias voltage to the bias electrode 2. When no bias voltage is applied,
Since the electron-hole pairs disappear by recombination before reaching the electrodes, no electric charge is stored in the capacitor 6 and saturation of the stored amount of the capacitor and increase in the voltage applied to the capacitor can be prevented. A two-dimensional radiation detector that does not cause withstand voltage breakdown is obtained.

In the two-dimensional radiation detector shown in FIG. 1, a large amount of X-rays enters one of the detection elements (pixels), and when the detected current value signal reaches a threshold value, the switch 20 is turned off and the semiconductor layer 1 is turned off. When the detection current value signal of any of the detection elements (pixels) exceeds the threshold value, an image signal corresponding to the incident X-ray dose cannot be obtained and the X-ray The strength cannot be evaluated.

That is, if the capacitor connected to the semiconductor layer has a sufficient capacity, when the X-ray with the dose b shown in FIG. 4B, the characteristic increases with time as shown in FIGS. 4A and 4B, and at X-ray irradiation end time t1,
The charge amount Q1 is stored in the capacitor for the X-ray amount A, and the charge amount Q2 is stored in the capacitor for the X-ray amount B.
Q2 is from the X-ray irradiation start time t0 to the X-ray irradiation end time t
It corresponds to an incident X-ray dose of up to 1.

However, in the two-dimensional radiation detector of FIG. 1, when a large amount of X-rays enter one of the detection elements (pixels) and the detected current value signal reaches a threshold value, the switch 20 is turned off at time t2. The application of the bias voltage to the bias electrode of the semiconductor layer is cut off, and the switch-off time t2 differs depending on the incident X-ray dose (t2 'for X-ray dose and t2 "for X-ray dose b), but the pixel reaches the threshold value. The amount of charge stored in the capacitor of the X-ray irradiation start time t0 regardless of the incident X-ray dose
From the time t2 when the switch is turned off to the charge amount q 'corresponding to the threshold value. Also, the amount of charge stored in the capacitors of the pixels other than the pixel that has reached the threshold value is smaller than the charge amount q ′ corresponding to the threshold value, as the incident X-ray amount from the X-ray irradiation start time t0 to the switch-off time t2. The corresponding charge amount is obtained, and neither the pixel reaching the threshold value nor the other pixels has the accumulated charge amount corresponding to the incident X-ray dose from the X-ray irradiation start time t0 to the X-ray irradiation end time t1.

As is apparent from FIG. 4, even when a large amount of X-rays are incident, the time t2 at which the switch is turned off differs according to the incident X-ray dose, and the amount of electric charge stored in the capacitor changes with the X-ray irradiation time. Since it is proportional, the accumulated charge amount q of the capacitor of each pixel from the X-ray irradiation start time t0 to the switch-off time t2 (the charge amount of the pixel exceeding the threshold value is the charge amount q ′ corresponding to the threshold value) The charge amount of the other pixels is changed from the X-ray irradiation start time t0 for each pixel to the switch-off time t.
The charge amount corresponding to the incident X-ray dose up to 2 is q 'or less), X-ray irradiation start time t0, X-ray irradiation end time t1,
If the time t2 when the switch is turned off is corrected as a parameter by the following equation, the accumulated charge amount Q of the capacitor corresponding to the incident X-ray from the X-ray irradiation start time t0 to the X-ray irradiation end time t1 is obtained. It can be seen that an image signal corresponding to is obtained. Q = (t1-t0) / (t2-t0) * q

FIG. 5 shows data obtained by correcting the data obtained by the two-dimensional radiation detector of FIG. 1 by the above equation so as to obtain an image signal corresponding to the incident X-ray dose. Components having the same function are denoted by the same reference numerals, and description thereof will be omitted. In FIG. 5, D is a two-dimensional radiation detector having the configuration of FIG. 1, 23 is an X-ray control circuit for turning on and off X-ray irradiation, 24 is a correction circuit including a CPU, and the correction circuit 24 is The output signal q corresponding to the read charge amount of the capacitor for each pixel read out, the switch-off time signal t2 measured by the circuit 24 with the switch-off signal from the control circuit 22, and the signal from the X-ray control circuit 23 X-ray irradiation start time signal t
0 and X-ray irradiation end time signal t1 and Q = (t1-
The image signal Q corresponding to the corrected incident X-ray amount is obtained by performing the calculation of (t0) / (t2−t0) * q.

In this configuration, when X-rays are irradiated at time t0, the X-rays transmitted through the subject pass through the bias electrode, enter the semiconductor layer, are absorbed in the semiconductor layer, and are transmitted through each pixel. An electron-hole pair (charge) corresponding to the dose is generated, and the generated charge is shifted by an electric field caused by a voltage applied to a bias electrode from a bias power source E,
An electric charge corresponding to the transmitted X-ray dose until the end of the X-ray irradiation at time t1 is accumulated in the capacitor for each pixel.

The charge signal stored in the capacitor is read out simultaneously by the switching element driving circuit 8 by sequentially driving the switching elements of the switching element matrix, line by line. The charge signal is supplied to the correction circuit 24 via the A / D converter 12, and a digital image signal for each pixel is output from the correction circuit 24.

If a large amount of X-ray exceeding the threshold does not enter any pixel of the detector D during X-ray irradiation, the control circuit 22 does not operate, and the correction is not made by the correction circuit 24.
From the X-ray irradiation time t0 to the X-ray irradiation end time t1, the correction circuit 24 outputs a digital image signal corresponding to the amount of charge accumulated in the capacitor of each pixel. When a large amount of X-rays exceeding the value enter, the detected current value signal from the current detector 21 exceeds the threshold value, and the control circuit 22 turns off the switch 20, and gives this switch-off signal to the correction circuit 24.

The CPU 24 measures the switch-off time t2 with the switch-off signal, and the switch-off time t2, A
The digital image signal q corresponding to the charge accumulated in the capacitor of each pixel from the X-ray irradiation start time t0 from the / D converter 11 to the switch-off time t2, and the X-ray irradiation start from the X-ray control circuit 23 From the time signal t0 and the X-ray irradiation end time signal t1, Q = (t1-t0) / (t2-t
0) * q is calculated, and a digital image signal Q corresponding to the incident X-ray dose from the corrected X-ray irradiation start time t0 to the X-ray irradiation end time t1 is output. The correction circuit 24
The output digital image signal Q is subjected to image processing by an image processing circuit (not shown) to be a two-dimensional X-ray image.

Next, an example of a method of manufacturing the two-dimensional radiation detector having the configuration shown in FIG. 1 will be described. Switching elements are provided in a matrix on a glass or other insulating substrate by thin film technology. Thereby, a switching element of a TFT, a MIM structure, or a double diode structure divided for each element, a signal electrode connected to the switching element, a driving line for driving the switching element (for example, in a column direction), A readout line (data line) (eg, a row direction) in which charges flow through the switching element is created. In general, switching elements are made of amorphous silicon or polysilicon, and for insulators, chemical materials such as nitride films, oxide films, and polyimides, and for metal materials, various materials such as titanium, aluminum, chromium, tantalum, and copper. Material is used.

A semiconductor layer is provided on the signal electrodes so as to cover the entire matrix of the signal electrodes. This semiconductor layer is formed by depositing a chalcogenide-based material such as selenium by evaporation, or by depositing silicon or cadmium telluride alloy with C.
It can be made in VD. In addition, it is also possible to use lead iodide, thallium bromide, or the like. The thickness of the semiconductor layer is preferably as large as possible to sufficiently absorb X-rays. The thickness is 1 to 5 mm for a low density material such as silicon, 300 to 1000 μm for selenium, and 100 to 300 μm for cadmium telluride alloy. Note that the semiconductor layer may be a continuous layer that covers the entire matrix-like signal electrode, or may be divided into a plurality of layers such as a plurality of layers obtained by combining several matrices.

Further, a bias electrode is provided on the semiconductor layer. When the incident line is light, ITO (indium tin oxide) is mainly used as the material of the bias electrode. However, when the incident line is an X-ray, the X-ray has a high penetrating power. A metal layer or an alloy layer of various thin films such as a gold thin film may be used. In addition, even if the bias electrode covers the entire surface of the semiconductor layer (FIG. 2A), FIG. 2B, FIG.
The divided bias electrodes may be divided into two or more as shown in (d). Each divided bias electrode is connected to a bias power source E via a series circuit of a switch 20 and a current detector 21 as shown in FIG. And a control circuit 22 that turns off the switch 20 when the detected current value reaches a threshold value.

In the above embodiment, the direct conversion type two-dimensional radiation detector in which X-rays are directly converted into electric charges in the semiconductor layer is used.
The present invention can also be applied to an indirect conversion type two-dimensional radiation detector in which a scintillator layer such as sI is provided to convert X-rays into light, and the converted light is converted into electric charges by a semiconductor layer. Further, in the embodiment, the switch-off time t2 is measured by the correction circuit (CPU). However, the detection current value detected by the current detection circuit is compared with a threshold value, and the control circuit that turns off the switch measures the switch-off time t2. A function may be provided so that the control circuit measures the switch off time and supplies the switch off time signal t2 to the correction circuit.

Further, in the embodiment, the correction circuit is a CPU and the correction is performed by digital operation. However, the amplifier connected to the readout line of the detector D or the digital image signal of the detector D is converted into an analog image signal. D / A to convert
A single amplifier may be provided downstream of the converter, and correction may be made by changing the gain of this amplifier. In this case, the gain of the amplifier is Q = (t1-t0) / (t2-
t0) * q. In the case where the bias electrode is composed of two or more divided bias electrodes, a switch, a current detector, and a control circuit for controlling the switch based on an output signal of the current detector are provided for each divided bias electrode, What is necessary is just to provide a correction circuit. By doing so, a radiographic image with higher image quality can be obtained.

[0034]

According to the two-dimensional radiation detector of the first aspect, when a large amount of radiation such as X-rays enters the semiconductor layer of the detector, the bias voltage applied to the bias electrode of the semiconductor layer is cut off. So saturate the capacitor, and
A two-dimensional radiation detector that does not cause the withstand voltage breakdown of the switching element due to the voltage rise of the capacitor can be obtained.

According to the two-dimensional radiation detector of the second aspect of the present invention, when the bias voltage applied to the bias electrode of the semiconductor layer is cut off, the detection is performed from the radiation irradiation start time until the bias voltage is cut off. Detector elements (pixels)
Based on the amount of charge accumulated in each capacitor, the amount of charge is corrected from the radiation irradiation start time to the radiation irradiation end time, so that an image signal corresponding to the incident radiation amount is obtained, and the capacitor is saturated. In addition, a radiation two-dimensional detector having a wide dynamic range can be obtained without causing a withstand voltage breakdown of a switching element connected thereto due to a voltage rise of a capacitor.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing one embodiment of the present invention.

FIG. 2 is a schematic view showing another embodiment of the present invention.

FIG. 3 is a schematic sectional view showing another embodiment of the present invention.

FIG. 4 is a characteristic diagram for explaining an operation.

FIG. 5 is a block diagram showing still another embodiment of the present invention.

FIG. 6 is a schematic diagram showing a configuration of a conventional two-dimensional radiation detector.

FIG. 7 is an equivalent circuit diagram of FIG. 6;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Semiconductor layer 2 ... Bias electrode 3 ... Signal electrode 4 ... Switching element matrix 5 ... Switching element (TFT) 6 ... Capacitor 7 ... Switching line 8 ... Switching element drive circuit 9 ... Readout line (data line) 10 ... Amplifier 11 ... Multiplexer 12 A / D converter 20 Switch 21 Current detector 22 Control circuit 23 X-ray control circuit 24 Correction circuit

Continued on the front page F term (reference) 2G088 EE01 FF02 FF14 FF18 GG21 JJ05 JJ33 JJ35 JJ37 KK40 4M118 AA02 AA08 AB01 BA05 CB05 CB07 CB11 CB14 FA06 FB13 FB16 GA10 5F088 AA09 AA05 BA05 BB03 FA05 CB03 KA10 LA08

Claims (2)

[Claims] 1. A radiation image or a light converted from a radiation image, having a bias electrode on one surface and a matrix-like signal electrode formed so that detection elements (pixels) are arranged vertically and horizontally on the other surface. A radiation two-dimensional detector comprising: a semiconductor layer that converts an image into a charge image; and a switching element matrix that two-dimensionally scans a signal electrode of the semiconductor layer and reads out the charge stored in the semiconductor layer. Connecting a bias electrode of the semiconductor layer to a bias power supply via a switch, detecting current flowing in a circuit interposed by the switch, and detecting a current value detected by the detecting means as a threshold value. A control circuit for turning off the switch when the radiation has reached
Dimensional detector. 2. The radiation two-dimensional detector according to claim 1, wherein a radiation irradiation start time t0, a radiation irradiation end time t1, and a time t2 when the switch is turned off by the control circuit are represented by the following equations. A two-dimensional radiation detector, further comprising: a correction unit that corrects an output signal of each pixel to obtain a correction signal Q. Q = (t1-t0) / (t2-t0) * q where q is an output signal of each pixel at the switch-off time t2.