JP3683987B2 - X-ray flat panel detector - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray flat panel detector used in an X-ray diagnostic apparatus for imaging an X-ray image transmitted through a human body.
[0002]
[Prior art]
Currently, imaging systems mainly used in X-ray imaging inspection include the following, for example.
[0003]
(1) In the X-ray imaging inspection by the screen / film system, an intensifying screen and an X-ray film are combined and X-ray imaging is performed in a state of being held in a case called a cassette.
[0004]
Further, in X-ray imaging, it is often used in combination with GRID (Bucky) that vibrates at high speed for the purpose of removing the influence of scattered radiation. Since this cassette is relatively small and light, with a thickness of several millimeters, if the subject cannot move, it can be brought to the bedside with a mobile X-ray generator and X-ray imaging can be performed on the spot. is there.
[0005]
Moreover, the cassette system seen in the X-ray bed has a magazine for storing unphotographed film, filmed film, and a film transport unit. At the time of X-ray photography, an unphotographed film is conveyed to a predetermined position where an X-ray grid and an intensifying screen are previously arranged, and photographing is performed by receiving X-rays.
[0006]
The photographed film can be observed as an X-ray image by performing development processing using an automatic developing device.
[0007]
However, in the current film, care must be taken in handling the film so that the film is not exposed or damaged until development is completed, and the operability is poor.
[0008]
In addition, a dedicated processor such as an automatic developing device is required, and since these dedicated processors use water and chemicals, the installation place is limited, and an image cannot be displayed immediately.
[0009]
(2) Next, computed radiography is an X-ray imaging method using a plate (imaging plate) coated with a stimulable luminescent material instead of using a conventional film as an X-ray detector.
[0010]
The imaging plate has a very wide dynamic range compared to the film, and can take an image in a wide dose range. When X-rays are exposed to the imaging plate, the energy level of electrons is increased by the energy of the X-rays, and the X-ray intensity distribution is stored as a latent image.
[0011]
Later, when high-order electrons are excited by laser irradiation, the energy at this time is detected as light. Since this light is proportional to the energy of X-rays absorbed by the imaging plate, an X-ray image can be obtained electrically.
[0012]
However, current imaging plates have poor resolution and are noisy. In addition, the imaging plate is scratched during use and its performance deteriorates. Furthermore, the reading device was expensive.
[0013]
(3) Further, there is a method of obtaining an X-ray image by combining an image intensifier (II) that converts X-rays into light and a television apparatus (II-TV system). In this method, the size of the X-ray input surface of the image intensifier is a size that can be photographed, and there are even about 16-inch fields of view.
[0014]
The X-ray image converted into light is formed once at the image intensifier output unit, and this output image is captured by a television camera via an optical system and output as an electrical image. In this method, an X-ray image can be observed in real time.
[0015]
However, this method has the disadvantage that the resolution is poor and the imaging system is larger than the film system.
[0016]
In recent years, it has portability such as the film-screen system and imaging plate, and has high resolution characteristics. I. -An X-ray flat panel detector using a thin film transistor (TFT) as a switching gate is considered as a next-generation X-ray imaging apparatus having real-time characteristics of a TV system (hereinafter referred to as an X-ray flat panel detector). Call).
[0017]
The following two configurations of the X-ray flat panel detector are currently considered. The X-ray flat panel detector of the first configuration includes a phosphor that converts X-rays into light on a detection surface on a flat plate, a photodiode array that converts the light into charges, a capacitor that accumulates charges, and a SW that reads charges. Composed.
[0018]
The X-ray flat panel detector of the second configuration reads a charge from a pixel portion made of a semiconductor layer that converts X-rays directly into charges on a detection surface on a flat plate, a charge accumulation portion made up of a capacitor that accumulates charges. It comprises a readout switch (referred to as readout SW).
[0019]
Both are electrical imaging devices that obtain an equal-magnification image like a film. I. -Like the TV device, it is excellent in immediate display of images, and electrical image storage is easy. Furthermore, since it is structurally thin, it can be replaced with a cassette using a film-screen system or an imaging plate.
[0020]
[Problems to be solved by the invention]
However, in the X-ray flat panel detector of the second configuration, the charge that increases in proportion to the X-ray incident amount of each pixel increases, 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 exposure.
[0021]
However, if the applied voltage exceeds a certain voltage during X-ray exposure, the read SW may be destroyed.
[0022]
This situation is thought to occur especially when more X-rays are exposed to the detector. Clinically, for example, when X-rays emitted from an X-ray tube do not pass through the subject's human body and directly enter the detector, or when X-ray exposure is accidentally performed for a long time For example, the charge increases, and a high voltage is applied to the charge storage portion. For this reason, since the voltage applied to the input side of the read SW exceeds a certain voltage, the read SW is destroyed.
[0023]
On the other hand, in order to obtain a good X-ray image, it is desirable that image information during X-ray exposure can be monitored in real time. As an implementation means thereof, an X-ray exposure monitor using a front-side daylighting type fiber scintillator in a film system; I. -A photomultiplier tube (PMT) or the like in a TV system. I. Output light monitors have been put into practical use.
[0024]
For X-ray flat panel detectors, a method such as combining front-illuminated fiber scintillators as in the film system can be considered. However, a dedicated detection mechanism is required, which increases the size of the entire detector and causes high costs. It was.
[0025]
SUMMARY OF THE INVENTION An object of the present invention is to prevent damage to a read SW while increasing its size and at a low price in an X-ray flat panel detector having a structure in which a high voltage can be applied to the read SW during X-ray exposure. An object of the present invention is to provide an X-ray flat panel detector capable of monitoring an X-ray exposure situation.
[0026]
[Means for Solving the Problems]
The present invention provides charge conversion means provided corresponding to each pixel of a plurality of pixels arranged on the detection surface and converting incident X-rays into charges, and the charge conversion provided corresponding to each charge conversion means. Charge storage means for storing the charge converted by the means, charge reading means provided corresponding to each charge storage means for reading out the charge stored by the charge storage means, and corresponding to each charge read means Provided and connected to the input side of the charge reading means, and when the applied voltage exceeds a predetermined voltage, the sweep means for sweeping out the charges accumulated in the charge accumulation means, and the sweep means sweeps out the charges. And a charge extracting means for extracting the collected charges to the outside.
[0027]
According to the present invention, when the applied voltage becomes equal to or higher than a predetermined voltage, the charge stored in the charge storage unit is swept out, and the charge swept out through the charge extraction unit is taken out to the outside. Therefore, the charge accumulation status can be monitored. For example, the presence or absence of an abnormal amount of X-ray exposure can be monitored.
[0037]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the X-ray flat panel detector of the present invention will be described.
[0038]
<Embodiment 1>
First, FIG. 1 shows an example of the X-ray flat panel detector of the present invention. In FIG. 1, an X-ray flat panel detector 10, a gate driver 19, read amplifiers 23a to 23e, and a multiplexer (MUX) 25 are provided.
[0039]
On the detection surface of the X-ray flat panel detector 10, a plurality of pixel portions 11 serving as charge conversion means composed of a plurality of pixels are arranged in a two-dimensional manner. For each pixel unit 11, a charge storage unit 13 as a charge storage unit, a TFT 17 as a charge readout unit, and a Zener diode 17 as a sweeping unit are provided.
[0040]
FIG. 2 shows a cross-sectional view of a single pixel of the X-ray flat panel detector. In FIG. 2, the image portion 11 is provided above the substrate 31 and between the upper electrode 33 and the lower electrode 35, and converts X-rays incident on the pixels into electric charges.
[0041]
Here, for example, amorphous-selenium (hereinafter referred to as a-Se) is used as the X-ray detection semiconductor that is the pixel unit 11. A high potential is applied to the upper electrode 33 of a-Se, and an electric field E is generated from the upper electrode 33 toward the lower electrode 35. Negative charges generated in a-Se by X-ray exposure are collected on the upper electrode 33, and positive charges are collected on the lower electrode 35.
[0042]
The lower electrode 35 is provided with the charge accumulating unit 13 composed of a capacitor for accumulating charges in series, and accumulates charges generated in a-Se.
[0043]
A TFT 15 is connected to the charge storage unit 13 through the lower electrode 35. The TFT 15 is a read SW that reads the charges accumulated by the charge accumulation unit 13 to the read amplifiers 23a to 23e.
[0044]
The TFT 15 operates as a switch by turning on the drain D and the source S when the gate control signal from the gate driver 19 is input to the gate G after the end of the X-ray exposure. After the X-ray exposure is completed, the charges having the X-ray image information accumulated in each pixel unit 11 are read out to the external read amplifiers 23a to 23e via the TFT 15.
[0045]
The gate driver 19 has four control lines 21a to 21d that output gate control signals, and each control line 21a to 21d is connected to the gate G of the four TFTs 15 in the row corresponding to its own control line.
[0046]
Each of the read amplifiers 23a to 23d is connected to the sources S of the four TFTs 15 in the column corresponding to its own read amplifier, reads out the electric charges of the four TFTs 15 in the corresponding column, and outputs them to the MUX 25.
[0047]
The MUX 25 converts the parallel output from each of the read amplifiers 23a to 23d into a serial output and sends it to an analog / digital (A / D) converter (not shown).
[0048]
Each of the Zener diodes 17 is connected to the drain D on the input side of the TFT 15 with a cathode C at one end, and breaks down at a predetermined voltage lower than the voltage that destroys the TFT 15, thereby causing the drain accumulation D 13 to the drain D of the TFT 15. Sweep the supplied charge to ground. Each Zener diode 17 is an element having a non-linear resistance action, and has a characteristic that a voltage breakdown occurs and a current flows when a reverse bias voltage of a certain amount or more is applied.
[0049]
Although the structure of the X-ray flat panel detector of the first embodiment has been described above, the structure of the X-ray flat panel detector excluding the configuration of the Zener diode 17 is, for example, “X-ray imaging using amorphous selenium: Feasibility of a”. flat panel'selfscanned detector for digital radiology, Wei Zhao et al., Med. Phys. 1995 ".
[0050]
In the description of this paper, a voltage of several thousand volts is applied to a-Se, and the voltage between a and Se decreases with the X-ray exposure, and conversely, the voltage between the charge storage units 13 increases. To do.
[0051]
Next, the operation of the first embodiment will be described with reference to an electrical equivalent circuit diagram of a single pixel shown in FIG. In this detector, during the X-ray exposure, the applied voltage 35 corresponds to the charge generated by the X-rays incident in the a-Se, and the applied voltage 35 is the pixel unit capacitance 37 of the pixel unit 11 which is a-Se. And the charge storage capacitor 39 of the charge storage unit 13.
[0052]
During the X-ray exposure, the TFT 15 serving as the readout SW is OFF, and the potential at the point A is applied to the TFT 15. A difference between the pixel side potential and the gate side potential of the TFT 15 or a difference between the pixel side potential and the readout amplifier 23a side potential is directly applied to the TFT 15 as a load.
[0053]
After the end of the X-ray exposure, the charge having the X-ray image information accumulated in each pixel unit 11 is read out to the read amplifier 23a via the TFT 15. In FIG. 3, the integration amplifier is configured such that the capacitor 41 is connected between the inverting input terminal and the output terminal of the read amplifier 23a.
[0054]
However, if the potential difference becomes more than a certain level (generally 50 to 100 V) during X-ray exposure, the TFT 15 will cause dielectric breakdown.
[0055]
For this reason, for each pixel of the detector, a Zener diode 17 which is a non-linear resistance element that breaks down at a constant voltage is arranged in parallel with the TFT 15, and one end is set to a reference potential (for example, GND).
[0056]
In addition, since the breakdown voltage of the Zener diode 17 is set lower than the breakdown voltage of the TFT 15, even if a potential higher than the breakdown voltage is generated at the point A, all current flows to the reference potential side via the Zener diode 17. It will flow. As a result, the TFT 17 serving as the read SW is not destroyed.
[0057]
In the first embodiment, the Zener diode 17 is described as an example of the element having the non-linear resistance action. However, a TFT having a similar function may be used.
[0058]
FIG. 4 shows an electrical equivalent circuit of a single pixel of an X-ray flat panel detector configured using TFTs as nonlinear resistance elements. In the example shown in FIG. 4, instead of the Zener diode 17, three TFTs 43a to 43c connected in series are provided between the drain D of the TFT 15 and the ground.
[0059]
In this case, the gate G and the source S of the TFT 43a are connected, the gate G and the source S of the TFT 43b are connected, and the gate G and the source S of the TFT 43c are connected.
[0060]
With the TFTs 43a to 43c having such a configuration, a diode is formed between the gate G and the drain D, and voltage breakdown occurs when a predetermined voltage is applied to the gate G. That is, since the same operation as the Zener diode 17 is performed, the TFT 17 is not destroyed. One or two TFTs having such a configuration may be used, or the gate G and the drain D of the TFT 43a may be connected instead of the connection between the gate G and the source S of the TFT 43a. Effects can be obtained.
[0061]
<Embodiment 2>
Next, a second embodiment of the X-ray flat panel detector of the present invention will be described. FIG. 5 shows an electrical equivalent circuit of a single pixel of the X-ray flat panel detector according to the second embodiment. In the first embodiment, the Zener diode 17 which is an element having a non-linear resistance action is arranged in parallel with the TFT 15 in order to prevent the TFT 15 serving as the read SW from being destroyed.
[0062]
In the second embodiment, a similar configuration is provided on the detector side, and the charge from one end of an element having a non-linear resistance action is read.
[0063]
The configuration will be described with reference to FIG. In the second embodiment, a Zener diode 17a having a cathode connected to the drain D of the TFT 15, a read amplifier 45 as a charge extracting means connected to the anode of the Zener diode 17a, and an inverting input terminal and an output of the read amplifier 45 A capacitor 46 provided between the terminals is provided.
[0064]
Since the other configuration is the same as that of the first embodiment shown in FIG. 3, the same parts are denoted by the same reference numerals and detailed description thereof is omitted.
[0065]
The zener diode 17 a sweeps out the charge supplied from the charge storage unit 13 to the drain D of the TFT 15 to the read amplifier 45 by breakdown at a predetermined voltage. The read amplifier 45 takes out the electric charge swept out from the Zener diode 17a to the outside.
[0066]
The breakdown voltage of the Zener diode 17a is set to a voltage corresponding to the saturation X-ray incident amount of each pixel. The electric charge swept out by the voltage breakdown of the diode 17a is taken out.
[0067]
In this case, the saturation X-ray incident amount of the pixel corresponds to the saturation charge amount of the pixel. Therefore, the breakdown voltage of the Zener diode 17a is set to be approximately the same as the voltage at the point A when the charge of the pixel is saturated.
[0068]
According to such a configuration, when X-ray incidence exceeding the saturation X-ray incidence amount occurs in the pixel, a voltage higher than the voltage at the time of pixel charge saturation is generated at point A. For this reason, the Zener diode 17a breaks down, and the charge is read from the Zener diode 17a to the read amplifier 45. Then, the output is displayed on an external display device or the like.
[0069]
Therefore, the presence or absence of an abnormal amount of X-ray exposure can be monitored. For example, when there is an X-ray incident amount exceeding the saturation X-ray incident amount, it is understood that an abnormal amount of X-ray exposure has been performed. In addition, it is possible to detect the presence or absence of a pixel that has generated a charge equal to or greater than the saturation charge amount without impairing the dynamic range of the detector.
[0070]
Next, an example of the method for processing one end of each element having a nonlinear resistance action according to the second embodiment will be described with reference to two.
[0071]
(Example 1)
First, FIG. 6 shows a configuration diagram of Example 1 of the processing method for one end of each element having nonlinear resistance action according to the second embodiment.
[0072]
In the X-ray imaging apparatus including the X-ray flat panel detector 10a shown in FIG. 6, the cathode of the Zener diode 17a is connected to the drain D of the TFT 15 for each pixel. The anodes of all the Zener diodes 17a are connected to one read amplifier 45.
[0073]
That is, this is a case where the anode which is one end of the Zener diode 17a of each pixel is combined into one.
[0074]
Further, the X-ray imaging apparatus is provided with a control device 47, an X-ray generator 49, and a display lamp 51. The control device 47 controls the X-ray generator 49 based on the output of the read amplifier 45. The X-ray generator 49 controls the X-ray dose based on a control signal from the controller 47. The display lamp 51 displays the output of the read amplifier 45.
[0075]
According to such a configuration, if even one pixel in the entire detector has an X-ray incident exceeding the saturation charge capacity, an output is produced.
[0076]
This output is sent to the control device 47, and the X-ray generator 49 cuts off X-rays corresponding to the output amount. In addition, the display lamp 51 may simply indicate that a saturated region exists.
[0077]
(Example 2)
Next, a second embodiment of the processing method for one end of each element having a nonlinear resistance action will be described. FIG. 7 shows a configuration diagram of Example 2 of the processing method for one end of each element having nonlinear resistance action according to the second embodiment.
[0078]
In the X-ray imaging apparatus including the X-ray flat panel detector 10a shown in FIG. 7, the entire 16-pixel detector is divided into, for example, four 2 × 2 blocks BK1, BK2, BK3, and BK4. The one end of the Zener diode 17a of each pixel is combined into one.
[0079]
For example, the anode of the Zener diode 17a of each pixel in the block BK1 is commonly connected to the readout amplifier 45-1, and the anode of the Zener diode 17a of each pixel in the block BK2 is commonly connected to the readout amplifier 45-2.
[0080]
The anode of the Zener diode 17a of each pixel in the block BK3 is commonly connected to the readout amplifier 45-3, and the anode of the Zener diode 17a of each pixel in the block BK4 is commonly connected to the readout amplifier 45-4.
[0081]
Outputs of the read amplifiers 45-1 to 45-4 are connected to the control device 47 and the display lamp 51. The configurations of the control device 47, the X-ray generation device 49, and the display lamp 51 are the same as those in the first embodiment.
[0082]
According to such a configuration, when there is X-ray incidence exceeding the saturation charge capacity in a certain block of the whole detector, the read amplifier corresponding to that block outputs an output. Therefore, it is possible to know the presence or absence of saturation for each of the four blocks.
[0083]
In clinical practice, depending on the imaging region, there is a region that is thin relative to X-rays outside the region of interest, and in order to obtain an appropriate density in the region of interest, it may be unavoidable even if part of the image is saturated. is there.
[0084]
By selecting and using only the signal information about the target portion from the information on the presence or absence of the four saturations and controlling the X-ray generator 49, finer exposure control becomes possible.
[0085]
Further, information indicating the halation region to the surgeon may be created from the four outputs, and the information may be displayed on the display lamp 51. Further, the number of divisions of 16 pixels is not limited to four, and may be any number. For example, a read amplifier may be provided for each pixel so as to know whether each pixel is saturated.
[0086]
Further, instead of displaying the information on the display lamp 51, the information may be notified by an alarm buzzer or the like.
[0087]
【The invention's effect】
According to the present invention, when the applied voltage becomes equal to or higher than a predetermined voltage, the charge accumulated in the charge accumulation unit is swept out, and the charge swept out through the charge extraction unit is taken out to the outside. Therefore, the charge accumulation status can be monitored. For example, the presence or absence of an abnormal amount of X-ray exposure can be monitored.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a first embodiment of an X-ray flat panel detector according to the present invention.
FIG. 2 is a cross-sectional view of a single pixel of an X-ray flat panel detector.
FIG. 3 is a diagram showing an electrical equivalent circuit of a single pixel.
FIG. 4 is a diagram showing an electrical equivalent circuit of a single pixel of an X-ray flat panel detector configured using a TFT as a nonlinear resistance element.
5 is a diagram showing an electrical equivalent circuit of a single pixel of the X-ray flat panel detector according to Embodiment 2. FIG.
6 is a diagram showing a configuration of Example 1 of a processing method for one end of each element having a nonlinear resistance action according to Embodiment 2. FIG.
7 is a diagram showing a configuration of Example 2 of a method for processing one end of each element having nonlinear resistance action according to Embodiment 2. FIG.
[Explanation of symbols]
10, 10a X-ray flat panel detector 11 Pixel unit 13 Charge storage unit 15, 43a-43c TFT (Thin Film Transistor)
17, 17a Zener diode 19 Gate drivers 21a-21d Control lines 23a-23e, 45 Read amplifier 25 MUX (multiplexer)
33 Upper electrode 35 Lower electrode 37 Pixel portion capacitance 39 Charge storage capacitance 41, 46 Capacitor 47 Control device 49 X-ray generator 51 Display lamp

Claims (4)

Charge conversion means provided corresponding to each pixel of the plurality of pixels arranged on the detection surface and converting incident X-rays into charges;
Charge storage means provided corresponding to each of the charge conversion means and for storing charges converted by the charge conversion means;
Charge reading means provided corresponding to each charge storage means for reading out the charges stored by the charge storage means;
Provided corresponding to the respective charge reading means, connected to said input side of the charge reading means, sweeping the applied charge voltage stored in the charge storage means when a Jo Tokoro voltage or sweeping Means,
An X-ray flat panel detector comprising: charge extraction means for extracting the electric charge swept out by the sweeping means to the outside. The X-ray flat panel detector according to claim 1, wherein the predetermined voltage is set in the vicinity of a voltage at the time of charge saturation of each pixel.   The charge extraction means is provided for each of the plurality of pixels divided into blocks, and each charge extraction means adds each charge swept from the sweeping means corresponding to each pixel in the plurality of pixels divided into blocks, A signal based on the added charges for a plurality of pixels is output to a display device or a control device that controls X-ray exposure as saturation presence / absence information indicating the presence / absence of a pixel that has entered an X-ray dose greater than or equal to the saturation X-ray incidence amount. The X-ray flat panel detector according to claim 2. The sweeping means, X-rays plane detector according to claim 1, wherein the non-linear element which generates a voltage breakdown when a voltage higher than a predetermined voltage is applied.

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