JP4208482B2 - Imaging device and X-ray diagnostic system using the imaging device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an imaging apparatus used for radiological medical equipment and the like.
[0002]
[Prior art]
Digitization is progressing in various fields of medicine. Also in the field of X-ray diagnosis, in order to digitize an image, a two-dimensional X-ray imaging apparatus that converts incident X-rays into visible light by a scintillator (phosphor) and further captures the visible light image with an image sensor. It has been developed. Advantages of the digitized X-ray imaging apparatus over the analog photographic technique include the following. These include filmlessness, expansion of acquired information through image processing, and creation of a database. In addition, it is useful in an urgent medical site that the captured image can be displayed instantaneously on the spot. Radiation means X-rays, α, β, γ rays, or high energy rays that can detect the internal structure of the subject, and light is an electromagnetic wave in a wavelength region that can be easily detected by a photoelectric conversion means such as a photodiode. Including visible light and infrared light. The case of X-rays as radiation will be described below.
[0003]
In the field of X-ray still images, as a two-dimensional X-ray imaging device, for example, a large-plate still image imaging device (flat) using amorphous silicon (a-Si) having a maximum of 43 cm □ for mammography and chest imaging. Panel detector) is made. An example of this type of technique is described in US Pat. No. 5,315,101. This conventional technique is shown in FIG. An image pickup device using an amorphous silicon semiconductor on a glass substrate is easy to obtain a large plate, and there is one that realizes a large plate X-ray image pickup device by tiling four panels. There is also a proposal for constructing a large X-ray imaging apparatus using a plurality of single crystal imaging elements (such as silicon imaging elements). An example of this type of technique is US Pat. No. 4,323,925, 5159455 shown in FIG. Examples of the single crystal imaging device include a CCD type imaging device using silicon, a MOS type imaging device, a CMOS type imaging device, and the like. A fiber optic plate (FOP) with a phosphor is used as the X-ray converter. The fiber optic plate is made of a material containing lead glass or the like that hardly transmits X-rays. In general, a single crystal silicon device such as a CCD is easily damaged by radiation such as X-rays and causes deterioration such as an increase in dark current and a decrease in photoelectric conversion efficiency. Therefore, a fiber optic plate is used as an X-ray shielding material. As a result, the X-ray resistance is increased and the reliability is improved. In a conventional CCD type image pickup device, MOS type image pickup device, or CMOS type image pickup device, a circuit (shift register, decoder, multiplexer, etc.) or memory circuit for driving the device is arranged on the outer periphery of the pixel region of the device. FIG. 13 shows a case where four CMOS image sensors are bonded. Since these circuits also cause malfunctions and deterioration when X-rays directly enter as in the pixel portion, they are generally configured to shield X-rays using lead or the like.
[0004]
In the field of X-ray animation, incident X-rays are converted into scintillators (phosphors) and I.D. I. A two-dimensional digital X-ray fluoroscopic apparatus has been developed that converts visible light by (image intensifier) and captures the visible light image with a TV camera using a CCD type imaging device. This is not a flat panel detector.
[0005]
In the field of medical X-ray diagnostics, which is becoming increasingly digitized, next-generation moving image capturing devices (such as fluoroscopy) that have been reduced in size, increased in sensitivity, and increased in speed by flat panels are expected.
[0006]
[Problems to be solved by the invention]
A radiation imaging apparatus (flat panel detector) using amorphous silicon (a-Si) can be easily made large, but it is difficult to achieve high sensitivity and high speed due to the characteristics of the amorphous silicon material. Therefore, it is limited to still images. A radiographic imaging device combining single crystal silicon and FOP can easily achieve high speed and high sensitivity, but it is inevitable to reduce the cost because a very expensive tapered FOP must be used for large plate. Therefore, there has been no X-ray imaging apparatus that is fully equipped with large plates, high speed, high definition, high sensitivity, and low cost. Furthermore, in order to reduce the influence of X-rays in single crystal silicon, there has been no driving considering the relationship between driving of X-ray pulses and X-ray noise caused by direct incidence.
[0007]
[Means for Solving the Problems]
To solve the above problem, Are generated sequentially at predetermined intervals, Transmitted through the subject radiation Converted light For converting to electrical signals Multiple pixels Each of the plurality of pixels includes a photoelectric conversion unit, a signal sample-and-hold circuit for storing the electric signal, and a noise sample-and-hold circuit for storing a noise signal. An imaging area; Arranged between pixels in the imaging region, Scanning means for scanning the imaging region, and a period in which the radiation is not generated, The electrical signal is output from the signal sample and hold circuit, and the noise signal is output from the noise sample and hold circuit. Said scanning means by The imaging area of scanning The And an image pickup apparatus characterized by comprising a control means for controlling.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
FIG. 1 is a diagram showing a configuration of an image sensor in which peripheral circuits such as horizontal and vertical shift registers as scanning means are formed in an effective pixel region. FIG. 2 is a diagram showing a layout of pixels in the effective pixel region, a shift register, a protection circuit, an external terminal, and the like. 3, 4, and 5 are diagrams for explaining an imaging module in which nine imaging elements are bonded together. FIG. 11 is a block diagram showing the configuration of the imaging apparatus according to this embodiment.
[0009]
As shown in FIG. 11, when a moving image of a subject is obtained, an X-ray pulse is generated from a pulse X-ray generator 100 that is a radiation source, and X-rays in the imaging panel 101 that have passed through the subject are made visible by a scintillator. Is converted to This light is converted into an electrical signal by an image sensor in the image pickup panel 101. The analog electric signal is converted into a digital signal by the A / D converter 102, then subjected to image processing by the image processing circuit 103, and stored in the storage unit 106 that stores a plurality of moving images. Then, an image is displayed on the display unit 105. The imaging element is driven by the sensor driving unit 107, and the pulse X-ray apparatus 100 is driven by the X-ray driving unit 108. Control of the pulse X-ray generator, each imaging device of the imaging module, and the image processing apparatus is performed by a controller 109 which is a control means. For the input operation to the controller, an operating device and an operation monitor are used.
[0010]
FIG. 4 shows an image pickup element portion of a large area image pickup module of 414 mm □ formed by sticking nine 138 mm □ image pickup elements in a tile shape.
[0011]
FIG. 5 shows an AA ′ cross section of FIG. Gd using europium, terbium, etc. as an activator ₂ O ₂ A scintillator plate made of a scintillator such as S or CsI is placed on the FOP. X-rays hit the scintillator and are converted into visible light. This visible light is detected by the image sensor. The scintillator is preferably selected so that its emission wavelength matches the sensitivity of the image sensor. The external processing board has a circuit for supplying power to the image sensor, a clock, and the like, and extracting and processing signals from the image sensor. TAB (Tape Automated Bonding) electrically connects each image sensor to an external processing board.
[0012]
The nine image sensors are bonded so that there is substantially no gap between the image sensors. Here, the fact that there is substantially no gap means that an image formed by nine image sensors cannot be lost between the image sensors. The clock of the image sensor, power supply input, and signal output from the pixel are performed with an external processing substrate disposed on the back side of the image sensor through a TAB connected to an electrode pad provided at the end of the image sensor. Even if the thickness of the TAB is sufficiently small with respect to the pixel size and passes through the gap between the image pickup elements, no defect on the image occurs.
[0013]
FIG. 3 shows a case where one image pickup device is taken out from the currently mainstream 8-inch wafer. The 8-inch wafer is an N-type wafer, and using this, a 138 mm □ CMOS-type image sensor is produced by a single process by a CMOS process.
[0014]
FIG. 6 shows a configuration diagram of a pixel portion constituting each pixel of the CMOS image sensor. Photodiode PD for photoelectric conversion, floating diffusion C for storing charge _FD , A transfer MOS transistor (transfer switch) for transferring the charge generated by the photodiode to the floating diffusion, a reset MOS transistor (reset switch) for discharging the charge accumulated in the floating diffusion, and a row selection MOS for selecting a row A transistor (row selection switch) and an amplification MOS transistor (pixel amplifier) functioning as a source follower are included.
[0015]
FIG. 7 shows a schematic diagram of the entire circuit with 3 × 3 pixels.
[0016]
The gate of the transfer switch is connected to φTX from a vertical shift register which is a kind of vertical scanning circuit, the gate of the reset switch is connected to φRES from the vertical scanning circuit, and the gate of the row selection switch is connected to φSEL from the vertical scanning circuit. It is connected.
[0017]
Photoelectric conversion is performed by a photodiode, and the transfer switch is in an OFF state during a light amount charge accumulation period. Charges photoelectrically converted by the photodiode are not transferred to the gate of the source follower constituting the pixel amplifier. The gate of the source follower constituting the pixel amplifier is initialized to an appropriate voltage by turning on the reset switch before starting the accumulation. That is, this is a dark level. Next or at the same time, when the row selection switch is turned on, the source follower circuit composed of the load current source and the pixel amplifier is activated, and the charge stored in the photodiode is changed by turning on the transfer switch here. And transferred to the gate of the source follower constituting the pixel amplifier.
[0018]
Here, the output of the selected row is generated on the vertical output line (signal output line). This output is sequentially read out to the output unit amplifier by driving a column selection switch (multiplexer) by a horizontal shift register which is a kind of horizontal scanning circuit.
[0019]
FIG. 2 shows a state in which unit blocks (units for selecting and driving one row) of a vertical shift register are arranged together with one pixel circuit in one pixel region (one cell). One pixel circuit is shown in FIG. The vertical shift register is a simple circuit composed of a static shift register and a transfer gate for generating a transfer signal φTX, a reset signal φRES, and a row selection signal φSEL. These are driven by signals from a clock signal line (not shown). The circuit configuration of the shift register is not limited to this, and an arbitrary circuit configuration can be adopted depending on various driving methods such as pixel addition and thinning readout. However, as in the present embodiment, the functional block is arranged together with the pixel circuit in one cell, and a shift register is provided in the effective pixel region, thereby realizing an imaging device in the entire effective pixel region.
[0020]
In this embodiment, a vertical shift register or n-to-2 ⁿ Vertical scanning circuit such as decoder, horizontal shift register and n-to-2 ⁿ A horizontal scanning circuit such as a decoder is arranged in each pixel region (cell) in the effective pixel region.
[0021]
Similarly, the present embodiment is characterized in that the common processing circuit is arranged in each pixel region (cell) in the effective pixel region. Here, the common processing circuit means a circuit that collectively processes a plurality of pixels such as a final signal output amplifier, a serial / parallel conversion multiplexer, a buffer, and various gate circuits.
[0022]
On the other hand, the individual circuit means a circuit that processes only one pixel, such as a photodiode, a transfer switch, a pixel selection switch, and a pixel output amplifier circuit.
[0023]
FIG. 1 shows the configuration (plan view) of the image sensor of this embodiment. In this embodiment, the vertical shift register and the horizontal shift register are arranged in the effective pixel area of the image sensor. One block of the shift register that processes one line in common is arranged so as to be within one pixel pitch. These blocks are arranged to form a series of vertical shift register blocks and to be a horizontal shift register block. These blocks extend linearly in the vertical and horizontal directions. The areas of the light receiving portions of the pixels having these shift register blocks are the same as the areas of the light receiving portions of the other pixels, so that variations in sensitivity are eliminated. Furthermore, the center of gravity (pixel center of gravity) of all the light receiving units is the same. A static shift register is used as the shift register. Various circuit configurations of the shift register can be applied by design. In this embodiment, a general circuit example is taken up.
[0024]
According to the present embodiment, since no dead space is generated around the image sensor, the entire surface of the image sensor is an effective pixel region.
[0025]
By arranging these image pickup elements in a tile shape so that there is substantially no gap, a large-plate image pickup device (flat panel detector) can be formed. An image of a large plate that is substantially seamless can be obtained.
[0026]
In a medical X-ray imaging apparatus, the size of a pixel may be as large as about 100 μm □ to 200 μm □, so that a sufficiently large aperture ratio can be realized even if a static shift register having a large number of components is arranged.
[0027]
The scanning means is not a shift register, but n-to-2. ⁿ A decoder can also be used. By connecting the output of the counter that sequentially increments to the input of the decoder, it becomes possible to scan sequentially as in the shift register, while at the same time, random scanning is performed by inputting the address of the area where the image is to be obtained to the input of the decoder. An image of an arbitrary region can be obtained.
[0028]
Since this embodiment uses a CMOS image sensor as the image sensor, it consumes less power and is suitable for configuring a large-sized image sensor.
[0029]
The reason why the multiplexer is built in the image sensor is to speed up the operation of the image sensor.
[0030]
A signal is taken out from the image pickup device via the electrode pad, and there is a large stray capacitance around the electrode pad. Therefore, by providing an amplifier in front of the electrode pad, signal transmission characteristics can be compensated.
[0031]
In the present embodiment, since the vertical and horizontal shift registers are arranged in the effective pixel region, the X-rays that have passed through the scintillator plate directly hit the shift register. X-rays are problematic because they damage elements in these circuits and cause errors and noise in the circuits.
[0032]
An example of an error is an insulating oxide film SiO ₂ This is a phenomenon in which charges are accumulated at the interface between silicon and silicon, resulting in threshold fluctuations and increase in leakage current. An example of damage is a defect generated on the pn junction surface, and this defect causes an increase in leakage current. Another example of the error is the same as an error (soft error) caused by the action of hot electrons known as a malfunction in the MOS type dynamic RAM. For example, in the driving circuit, noise is generated in the reset circuit, and the scanning is temporarily stopped or the shift pulse is shifted. In other peripheral circuits, noise is often the main cause. Hot electrons generated by an electric field are likely to occur in a short channel structure in which the electric field is high. However, hot electrons generated by an X-ray are generated regardless of the size. Tends to be unstable. As noise, an electron / hole pair is generated in silicon by X-rays entering single crystal silicon. There is noise generated when these carriers enter the circuit, and it is added to the inherent system noise of the circuit. Such a phenomenon occurs not only in the shift register but also in the case of using a circuit such as a decoder or a common amplifier that may be directly irradiated with X-rays.
[0033]
By using FOP, the amount of X-rays (X-ray photon number) that directly enters the image sensor can be reduced to several tenths or less. Since X-ray damage is proportional to the total amount of absorbed X-rays, FOP can significantly reduce X-ray damage. On the other hand, errors and noise due to X-rays depend not only on the total amount of X-rays but also on the energy of the X-rays. The energy of X-rays generated at a tube voltage of 80 kVp is very large, about 80 keV at maximum and about 60 keV on average. If even one high-energy X-ray photon is absorbed, tens of thousands of electron / hole pairs are generated. When they enter the light receiving section, they become spike-like signals, and at the same time, fluctuations in the signals are added as noise. When it enters a capacitor, signal line, or transistor other than the light receiving portion, it is added as a noise component. Intrusion into a logic circuit such as a shift register causes an operation error. In other words, with FOP alone, damage can be reduced, but it is difficult to reduce errors and noise.
[0034]
In the present invention, an X-ray imaging apparatus resistant to noise caused by direct X-rays can be realized by considering the following driving. FIG. 8 is a timing chart showing the operation of the imaging apparatus of the present embodiment. The operation will be described with 3 × 3 pixels.
[0035]
At T1, the reset signal φRES becomes high level, and the reset MOS transistors of all the pixels are turned on, so that the floating diffusion C _FD Is reset. An X-ray pulse for imaging a subject is irradiated and the photodiode is exposed. By turning on the transfer switch by setting the transfer signal φTX to the high level for all the pixels at once, the floating diffusion C formed in the gate portion of the source follower that constitutes the pixel amplifier is stored in the photodiode. _FD To complete transfer. When the row selection signal SEL1 becomes high level, the row selection MOS transistor of the corresponding row is turned on, and the imaging signal is read to the signal output line via the amplification MOS. The image signals for one row read out in this way are read out of the image sensor through the common amplifier when the column selection signals φSH1, φSH2, and φSH3 are sequentially turned on by the horizontal shift register. Similarly, the signal of each row is read out. In this embodiment, the X-ray pulse is applied between the reset signal φRES and the transfer signal φTX. While the X-rays are irradiated on the entire surface of the FOS pixel area, the horizontal shift register and the vertical shift register installed in the pixel area do not generate drive signals, so that they do not malfunction. Further, noise due to X-rays is not added to these circuits.
[0036]
These operations are performed by the vertical and horizontal shift registers installed in the pixel region, and X-rays are not irradiated while these circuits are driven, and they do not malfunction. During this time, no carriers are generated by X-rays entering the silicon bulk, and no noise is added during circuit operation. In order to perform pixel addition for each pixel, an addition switch for adding and outputting pixel signals may be provided. In such a case, a common processing circuit is provided in the pixel region in order to process pixels in common. The common processing circuit performs an operation such as addition when the X-ray is off. At the time of addition, noise due to X-rays is not directly added to the signal after addition, and the common processing circuit does not malfunction.
[0037]
In the embodiment described above, a phosphor is used to convert X-rays into visible light. However, a general scintillator, that is, a wavelength converter may be used. Further, even if there is no phosphor, the photoelectric conversion element itself may directly detect radiation and generate a charge.
[0038]
Moreover, although the case where X-rays are used has been described as an example, radiation such as α, β, and γ rays can be used.
[0039]
(Second Embodiment)
Next, a second embodiment of the present invention will be described. The image sensor used in the second embodiment has the same basic configuration as that of the first embodiment, but the circuit configuration of one pixel is different from that of the first embodiment. FIG. 9 shows a one-pixel circuit according to the second embodiment of the present invention. In the present embodiment, kTC correction in the photoelectric conversion unit is performed in the pixel, and sensitivity switching means is provided in the pixel, so that still image shooting and high-speed moving image shooting are realized by mode switching.
[0040]
Here, the specific conditions required for the photoelectric conversion unit in the image sensor for both still image shooting and moving image shooting will be described. The irradiation X-ray dose at the time of moving image shooting is about 1/100 of that at the time of still image shooting, and the amount of X-ray photons at most per pixel (actually incident on this pixel is the visible light converted from this X-ray). There is a need for maximum sensitivity as an image sensor. However, there is no problem with the dynamic range. Further, a reading speed of 60 to 90 frames / second is required. The pixel resolution may be as coarse as 200 μm □ to 400 μm □. On the other hand, a dynamic range close to 80 dB is required during still image shooting. The pixel resolution needs to be 100 μm □ to 200 μm □. There has been no image sensor that satisfies these specifications at the same time.
[0041]
Therefore, in the present embodiment, an image sensor satisfying these specifications is realized by adopting a pixel circuit configuration as shown in FIG. 9 in the CMOS image sensor. In FIG. 9, PD is the same embedded photodiode used in a CCD or the like as a photoelectric conversion unit. The buried photodiode has a high impurity concentration on the surface. ⁺ By providing a layer, SiO ₂ This suppresses dark current generated on the surface. The capacitance C of the photodiode PD _PD Is designed to be minimized to obtain maximum sensitivity when shooting movies. Photodiode as described below _PD Decreasing the capacity reduces the dynamic range. Since a dynamic range is insufficient at the time of still image shooting in which the irradiation X-ray dose is 100 times or more compared with that at the time of moving images, a capacitor C1 for expanding the dynamic range is provided in parallel with the photodiode FD.
[0042]
M1 is a changeover switch for switching between a still image mode (high dynamic range) and a moving image mode (high sensitivity mode). Floating diffusion (floating diffusion region) capacitance C for storing charge _FD (Not shown) is also designed to have a minimum capacity so that the maximum sensitivity is obtained during moving images. The floating diffusion (floating diffusion region) is formed connected to the gate portion of the amplification MOS transistor M4. M2 is a reset MOS transistor (reset switch) for discharging charges accumulated in the floating diffusion, M3 is a selection MOS transistor (selection switch) for selecting the pixel amplifier 1, and M4 is an amplification MOS functioning as a source follower. This is a transistor (pixel amplifier 1).
[0043]
A clamp circuit, which is a feature of the present embodiment, is provided at the subsequent stage of the pixel amplifier 1. This clamp circuit removes kTC noise generated in the photoelectric conversion unit. C _CL Is a clamp capacitor, and M5 is a clamp switch. A sample and hold circuit is provided after the clamp circuit as in the first embodiment. M6 is a selection MOS transistor (selection switch) for selecting the pixel amplifier 2, and M7 is an amplification MOS transistor (pixel amplifier 2) that functions as a source follower. M8 is a sample MOS transistor switch constituting a sample hold circuit for storing optical signals, and CH1 is a hold capacitor.
[0044]
M9 is a selection MOS transistor (selection switch) for selecting the pixel amplifier 3, and M10 is an amplification MOS transistor (pixel amplifier 3) functioning as a source follower. M11 is a sample MOS transistor switch constituting a sample and hold circuit for accumulating noise signals, and CH2 is a hold capacitor. M12 is a selection MOS transistor (selection switch) for selecting the pixel amplifier 3, and M13 is an amplification MOS transistor (pixel amplifier 3) functioning as a source follower.
[0045]
In the present embodiment, in order to remove FPN due to thermal noise, 1 / f noise, temperature difference, and process variation in the pixel amplifier, these optical signals and the in-pixel sample and hold circuit for noise are used.
[0046]
Next, the configuration of the pixel portion will be described. In this embodiment, since the pixel is as large as 160 μm □, the capacitance C with an appropriate aperture ratio (photodiode area). _PD There is a limit to making it smaller. Capacitance C can be obtained by reducing the electrode area while keeping the photodiode area unchanged. _PD However, in this method, the efficiency of collecting charges on the electrodes is lowered, and it becomes difficult to completely transfer the signal charges to the floating diffusion by the transfer switch. In this embodiment, the design is such that complete transfer is not performed, no transfer switch is provided, and a photodiode and a floating diffusion are directly connected to form a photoelectric conversion unit. In addition, the capacitance C of the photodiode is set so that the maximum sensitivity is obtained when shooting a movie. _PD And the capacity C of the floating diffusion _FD Is designed to be minimal.
[0047]
In the present embodiment, since the transfer is not complete, kTC noise is generated when the photoelectric conversion unit is reset. However, removing this kTC noise (reset noise) in terms of the circuit can increase the S / N ratio of the photoelectric conversion device. It becomes an important point. Therefore, in this embodiment, a clamp circuit is provided for each pixel. It is known to use a clamp circuit to remove kTC noise. If the pixel size is relatively small, such as 50 to 100 μm □, and complete transfer is possible, kTC noise does not occur in the photoelectric conversion unit, so this is not the case.
[0048]
However, in order to obtain an image sensor that uses both the still image mode and the moving image mode, it is necessary to remove kTC noise even in the still image mode, and it is essential to provide a clamp circuit in the pixel. In the present embodiment, a clamp circuit is provided before the sample-and-hold circuit for batch exposure so that kTC noise can be removed even in the batch exposure moving image mode.
[0049]
In order to increase the dynamic range of the photodiode for still image shooting, the capacitance C _PD Is better, but the signal voltage will decrease, and the S / N will decrease. In order to expand the dynamic range at the time of still image shooting while maintaining the highest sensitivity at the time of moving image shooting, a sensitivity (dynamic range) switching circuit is provided, and a capacitor and a changeover switch are provided in each pixel in this embodiment. Since the capacity increases during still image shooting, the S / N deteriorates, but in order to improve the S / N, a clamp circuit that specifically removes kTC noise is necessary.
[0050]
FIG. 10 is a timing chart showing the operation of the pixel portion when the present embodiment is driven in the moving image mode. The drive timing of this image sensor will be described with reference to FIG.
First, the signals φEN and φCHG from the vertical shift register VSR are set to the high level for all the pixels, the selection switches M3 and M6 are turned on, the constant current source to the amplification transistor is turned on, and the signal φRES is set to the high level to charge the photodiode. To the reset potential. Next, the signal φCL is set to the high level to turn on the MOS transistor M5, whereby one end of the clamp capacitor is biased to the clamp potential. In this way, kTC noise (reset noise) generated by resetting the charge of the photodiode is accumulated in the clamp capacitor. When the signal φTN is set to the high level, the clamp potential of the clamp capacitor is accumulated in the noise sample hold capacitor CH2. The above is the noise signal accumulation operation.
[0051]
Next, exposure is performed by X-ray pulse irradiation, and photoelectric charges are accumulated in the photodiode (this is an exposure operation). Thereafter, when the signal φEN is set to a high level, the potential at one end of the clamp capacitor changes by a signal component corresponding to the photocharge. That is, since the kTC noise component is accumulated in the clamp capacitor, the potential of one end of the clamp capacitor varies by the potential obtained by subtracting the kTC noise component from the imaging signal including the kTC noise component. In order to read out the imaging signal with the kTC noise canceled, the signal CHG and the signal φTS are set to the high level. By doing so, the imaging signal (optical signal) from which kTC noise has been canceled is accumulated in the signal sample-and-hold capacitor. The above is the optical signal accumulation operation.
[0052]
The above series of operations is repeated. Here, the exposure time of the image sensor is after the clamp potential is transferred to the capacitor, that is, from when the signal EN becomes low level until the signal φRES becomes high level and the reset MOS transistor is turned on. In the present embodiment, attention is particularly paid to the relationship between the timing of the X-ray pulse and the drive timing of the shift register installed in the pixel region. In this embodiment, X-rays are not irradiated during signal readout. As shown in FIG. 10, X-ray irradiation is performed after the signal readout is finished and before the next φEN is turned on. That is, while the X-ray pulse is not irradiated, the signal φSEL is set to a high level for each row, and the signal φSELH is set to a high level for each column, whereby a noise signal and an optical signal are output. These drive signals are generated by vertical and horizontal shift registers, and X-rays are not irradiated while these shift registers generate signals. In other words, while the X-ray pulse is irradiated and the photodiode is exposed, the element in the pixel is not driven by the shift register. Therefore, when the X-ray pulse directly enters the image sensor, an error (malfunction) does not occur in the shift register and noise does not occur during signal transfer.
[0053]
The signal transferred to the noise signal output line and the optical signal output line is subjected to a (signal-noise) subtraction process by a subtraction output amplifier (not shown) connected to the noise signal output line and the optical signal output line. At this time, the optical signal and the noise signal are taken into the sample hold circuit from the pixel amplifier 2 with a very fast time difference, so that 1 / f noise having a large value at a low frequency can be removed, and a high frequency component can be ignored. Further, with this time difference, there is no variation in the threshold value Vth due to the temperature difference of the output stage source follower. The output charge stored in the hold capacitor is obtained by continuously obtaining the output in the case of both resetting and signal charge input for one pixel amplifier in time. By taking the difference, FPN due to thermal noise, 1 / f noise, temperature difference, and process variation in the pixel amplifier can be removed.
[0054]
On the other hand, in the still image mode, the same operation as described above is performed when the signal φSC is set to the high level and the capacitor C1 is connected in parallel to the photodiode PD. In this case, the capacity C1 includes the capacity C _FD Because it has a capacity that is nearly 10 times larger than the above, a wide dynamic range can be realized. Further, kTC noise in the photoelectric conversion unit can be removed for each pixel by a clamp circuit. Further, by providing a sample hold circuit for storing optical signals and noise signals in the pixel, it is possible to remove FPN caused by thermal noise, 1 / f noise, temperature difference, and process variation in the pixel amplifier. Thereby, in the moving image mode, it is possible to realize high-speed and high-sensitivity moving image shooting that is temporally and spatially seamless with nine image sensors. On the other hand, the still image mode can realize still image shooting with high sensitivity and high dynamic range.
[0055]
These operations are performed by the vertical and horizontal shift registers installed in the pixel region, and X-rays are not irradiated while these circuits are driven, and they do not malfunction. During this time, no carriers are generated by X-rays entering the silicon bulk, and no noise is added during circuit operation. In order to perform pixel addition for each pixel, an addition switch for adding and outputting pixel signals may be provided. In such a case, a common processing circuit is provided in the pixel region in order to process pixels in common. The common processing circuit performs an operation such as addition when the X-ray is off. At the time of addition, noise due to X-rays is not directly added to the signal after addition, and the common processing circuit does not malfunction.
[0056]
In this embodiment, signal and noise sample and hold circuits are provided. When signals are accumulated in these circuits, if X-rays are irradiated, noise may be generated in these circuits and the image quality may be deteriorated. Therefore, it is preferable not to irradiate X-ray pulses while signals are accumulated in these sample and hold circuits.
[0057]
In each of the embodiments described above, a phosphor is used to convert X-rays into visible light. However, a general scintillator, that is, a wavelength converter may be used. Further, even if there is no phosphor, the photoelectric conversion element itself may directly detect radiation and generate a charge.
In each embodiment, the case where X-rays are used has been described as an example, but radiation such as α, β, and γ rays can be used.
[0058]
As described above, in the first and second embodiments, scanning means such as vertical and horizontal shift registers capture images during a period in which no radiation pulse is generated from the pulse X-ray generator 100 at a predetermined interval. By having the controller 119 for controlling the sensor driving unit 107 and the X-ray driving unit 108 so as to scan the region, it is possible to obtain a good image.
[0059]
(Third embodiment)
FIG. 12 shows an application example of the imaging apparatus described in the first and second embodiments to an X-ray diagnostic system.
[0060]
X-rays 6060 generated by the X-ray tube 6050 pass through the chest 6062 of the patient or subject 6061 and enter a radiation imaging apparatus 6040 including a scintillator, an FOP, an imaging device, and an external processing board. This incident X-ray includes information inside the body of the patient 6061. The scintillator emits light in response to the incidence of X-rays, and this is photoelectrically converted by the image sensor to obtain electrical information. This information is converted into digital data, processed by an image processor 6070, and can be observed on a display 6080 in a control room.
[0061]
This information can be transferred to a remote place by transmission means such as a telephone line 6090, and can be displayed on a display 6081 such as a doctor room in another place or stored in a storage means such as an optical disk, and diagnosed by a remote doctor. Is also possible. It can also be recorded on the film 6110 by the film processor 6100.
[0062]
【The invention's effect】
As described above, according to the imaging apparatus according to the present invention, it is possible to obtain a high-quality moving image.
[Brief description of the drawings]
FIG. 1 is a plan view showing a layout of an image sensor.
FIG. 2 is a diagram illustrating a relationship between one pixel circuit in an image sensor and a unit block of a shift register.
FIG. 3 is a plan view showing an image sensor and a wafer that is the source of the image sensor.
FIG. 4 is a plan view showing an arrangement of image pickup elements and an arrangement of scanning circuits in the image pickup module.
5 is a cross-sectional view showing a configuration of an imaging module, and shows a cross section taken along line AA ′ of FIG.
FIG. 6 is a one-pixel circuit diagram of the image sensor according to the first embodiment of the present invention.
FIG. 7 is an equivalent circuit in the image sensor according to the first embodiment of the present invention.
FIG. 8 is an operation timing in the first embodiment of the present invention.
FIG. 9 is a one-pixel circuit diagram of an image sensor according to a second embodiment of the present invention.
FIG. 10 is an operation timing in the second embodiment of the present invention.
FIG. 11 is a block diagram of an imaging apparatus.
FIG. 12 is a conceptual diagram showing a configuration of a radiation imaging system.
FIG. 13 is a diagram showing a first conventional technique.
FIG. 14 is a diagram showing a second conventional technique.
FIG. 15 is a diagram showing a third conventional technique.
[Explanation of symbols]
100 pulse X-ray equipment
101 Imaging module
107 Sensor driver
108 X-ray drive unit
109 Controller

Claims (7)

A plurality of pixels for sequentially converting light generated by sequentially converting the radiation transmitted through the subject into electrical signals at predetermined intervals, and each of the plurality of pixels includes a photoelectric conversion unit and the electrical signal. An imaging region having a signal sample-and-hold circuit for storing and a noise sample-and-hold circuit for storing a noise signal ;
A scanning means disposed between the pixels in the imaging region and for scanning the imaging region;
The period during which the radiation is not generated, said from the signal sample-and-hold circuit electrical signal is output, and so the noise signal from the noise sample-and-hold circuit is output, the imaging region by the scanning unit Control means for controlling scanning;
An imaging apparatus having 2. The method according to claim 1, wherein each of the plurality of pixels includes an amplifying unit that amplifies and outputs a signal from the photoelectric conversion unit, and a reset unit that resets an input unit of the amplifying unit. The input unit of the amplifying unit is reset during a period in which the radiation is not generated, and the noise signal after the reset is read from the amplifying unit to the noise sample hold circuit, and the electric signal generated by the photoelectric conversion unit An imaging apparatus , wherein scanning of the imaging region by the scanning unit is controlled so that a signal is read from the amplification unit to the signal sample hold circuit . 3. The pixel according to claim 2 , wherein each of the plurality of pixels further includes a clamp circuit provided at a subsequent stage of the amplifying unit, and the signal sample hold circuit and the noise sample hold circuit are provided at a subsequent stage of the clamp circuit. imaging apparatus characterized by being. In any one of claims 1 to 3, wherein the control means, characterized in that the radiation in the period has not occurred, and controls the scanning means to sum the signals from the plurality of the photoelectric conversion portion An imaging device. 4. The scanning means according to claim 2 , wherein the scanning means is a vertical scanning circuit for supplying a driving signal to the reset means, a common processing circuit for sequentially reading the output signals, and driving the common processing circuit. An image pickup apparatus comprising: a horizontal scanning circuit, wherein the control unit drives the vertical scanning circuit and the horizontal scanning circuit during a period in which the radiation is not generated .   The signal processing unit according to claim 1, a signal processing unit that processes a signal from the imaging region, a recording unit that records a signal from the signal processing unit, and a signal from the signal processing unit. An image pickup apparatus having display means for displaying. X-ray diagnostic system comprising: the image pickup apparatus, comprising: a processor for recording the information recording medium from said image pickup device, to Claim 6.

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