Patent Application
20070040099
1. Field of the Invention
The present invention relates to a radiation image pickup apparatus and a control method thereof, each suitable for being used for a medical diagnosis, an industrial nondestructive inspection and the like. In addition, it is supposed that electromagnetic waves of X rays, γ rays and the like, and α rays and β rays are included in radiations.
2. Description of the Related Art
Recently, a radiation image pickup apparatus using a non-single crystal thin film semiconductor such as amorphous silicon or the like as a solid state image pickup device to be able to radiograph a still image has been put to practical use. Enlargement in area exceeding 40 cm square which can cover the size of the chest of a human body has been realized using a producing technique of amorphous silicon thin film semiconductor. Because the manufacturing process thereof is comparatively easy, it is expected that an inexpensive detection apparatus will be provided in the future. Moreover, because the amorphous silicon can be produced to be a thin glass plate having a thickness of 1 mm or less, the amorphous silicon has an advantage capable of being produced as a detector having a very thin thickness. Moreover, because it is possible to implement radiographing a still image and a moving image with one apparatus, it is unnecessary to prepare two apparatus dedicated for them in total.
Such a radiation image pickup apparatus includes a photoelectric conversion circuit equipped with a plurality of conversion elements converting a radiation such as an X ray into an electric signal and switching elements arranged in a matrix, and a reading circuit for reading the electric signal from the photoelectric conversion circuit. Here, the conversion elements may be made of a material converting a radiation into an electric signal directly, or may be composed of wavelength converters each converting a radiation into a visible light and photoelectric conversion elements each converting the converted visible light into an electric signal.
When an image is radiographed by a radiation image pickup apparatus including solid state image pickup devices using the amorphous Si, an actually read image includes offsets generated by the photoelectric conversion circuit and the reading circuit. Accordingly, it is necessary to remove the offset components included in the signal value.
It is known to perform the correction of offset components (hereinafter referred to as offset correction) by subtracting the signal values (dark signal values) of a dark output image including the offset components such as fixed pattern noises, dark component and the like from the signal values of an image acquired by an exposure to a radiation. When it is supposed that the values obtained by the subtractions are corrected values, each of the corrected values can be acquired by the formula “corrected value=signal value−offset component.” In addition, the offset components can be acquired by performing signal reading (reading operation for correction) from a converting circuit without performing any exposure to a radiation.
For instance, in moving image radiographing, an offset component has been previously acquired, and the signal value has been stored in a memory or the like before an exposure to a radiation. And, corrected values are acquired by subtracting the stored offset components from the signal values acquired in the following frames. Hereupon, it is supposed that an operation of acquiring an offset component one time is counted as one frame similarly to an operation of acquiring a signal value.
By the conventional method, first, an offset component is acquired just after a moving image radiographing start, i.e. just after an instruction of an exposure to a radiation. And, the acquired offset component is stored in an offset memory. After the completion of the acquirement of the office component, the exposure to a radiation is begun to acquire a signal value output. And, the acquired signal value is stored in a signal value memory. After that, the previously acquired offset component is subtracted from the signal value in the signal value memory with a subtraction circuit to acquire a corrected value. In the following frames, the signal values are similarly acquired. However, the signal value memory used in the previous frame may be overwritten by the signal at this time.
However, an exact offset component cannot be acquired in the moving image radiographing including an exposure to a radiation by the conventional method, and the conventional method has a problem of the occurrence of an error of an actually acquired corrected value. It is known that an actual signal value by a moving image radiographing includes an image lag component in addition to the image signal value output component and the offset component.
Hereupon, the image lag means a component which remains apart from an image signal value output component even if the signal value is corrected by the offset correction. Consequently, the signal value can be expressed by the formula “signal value=image signal value output component +offset component+image lag.” If the formula is rewritten, the formula “corrected value=signal value−offset component=image signal value output component+image lag” is acquired. As mentioned above, because the image lag remains in the corrected value by the prior art, the moving image deteriorates.
When a positive image lag is generated after an offset acquisition in the conventional offset correction, a larger corrected value is output by the quantity of the image lag than the actual corrected value. That is, the conventional apparatus has a problem of the deterioration of image quality in moving image radiographing owing to the influences of the image lag.
Moreover, when the standard deviation of random noises included in a signal value acquired by a normal reading operation is denoted by a letter σ1, and the standard deviation of random noises included in an offset component acquired by a reading operation for correction is denoted by a letter σ2, the standard deviation σt of random noises included in a corrected value is expressed by the following formula 1.
σ1=√σ12+σ22 (1)
And, the random noises increase by the random noises included in the offset component as compared with those before the correction.
In addition, a technique of reducing the random noises is disclosed in, for example, U.S. Pat. No. 6,904,126. The U.S. Pat. No. 6,904,126 discloses a correction method of subtracting an average value of a plurality of dark outputs before an exposure to a radiation or an average value of a plurality of dark outputs after an exposure to a radiation from an image signal generated by the exposure to a radiation.
However, the method of subtracting the average value of the plurality of dark outputs before the exposure to a radiation cannot correct the random noises generated by the exposure to a radiation even if the method can correct offset components. On the other hand, the method of subtracting the average value of the plurality of dark outputs after the exposure to a radiation is lead to perform the steps of reading a plurality of dark outputs, averaging the read dark outputs and the like after the acquisition of the image information even if the method can reduce the random noises. Consequently, the time up to the output of the image is delayed. Hence, the delay of the image output becomes a problem especially at the time of the acquisition of a moving image. Moreover, also the frame rate thereof is lowered. Furthermore, the U.S. Pat. No. 6,904,126 has no concept of the image lag, and cannot sufficiently correct the image lag together with the random noises. In particular, when an amorphous material is used as a photoelectric conversion layer, carriers generated by photoelectric conversions are trapped at a trap level in the amorphous material. Consequently, many image lags are generated.
The present invention has been made in view of the problems mentioned above, and it is an object of the present invention to provide a radiation image pickup apparatus capable of reducing offset components and random noises without lowering the frame rate thereof, and a control method thereof.
The inventor of the present application has hit upon each aspect of the present invention which will be shown in the following as a result of zealous examinations in order to settle the problems mentioned above.
A radiation image pickup apparatus according to the present invention includes: a detection unit in which a plurality of conversion elements, each converting a radiation into charges, is two-dimensionally arranged; a driving circuit driving the detection unit; a reading circuit reading an electric signal based on the charges from the detection unit; a control unit selectively executing a first reading operation reading a first signal value by driving the detection unit, on which the radiation has been irradiated, a second reading operation reading a second signal value by driving the detection unit without being irradiated by any radiations before the first reading operation, and a third reading operation reading a third signal value by driving the detection unit without being irradiated by any radiations after the first reading operation; and a signal processing unit processing the electric signal output from the reading circuit, wherein the signal processing unit subtracts a signal value acquired by averaging processing of the second signal value and the third signal value from the first signal value.
A radiation image pickup system according to the present invention includes the radiation image pickup apparatus and a radiation generating unit, wherein the control unit controls operations of the radiation generating unit and the radiation image pickup apparatus to read a radiation image having transmitted a subject.
A control method of a radiation image pickup apparatus according to the present invention includes: a first reading step of reading a first signal value from a detection unit with a reading circuit by driving the detection unit irradiated by a radiation with a driving circuit, the detection unit including a plurality of conversion elements, which is two-dimensionally arranged and converts the radiation into charges; a second reading step of reading a second signal value from the detection unit with reading circuit by driving the detection unit with the driving circuit without being irradiated by any radiations before the first reading step; a third reading step of reading a third signal value form the detection unit with the reading circuit by driving the detection unit with the driving circuit without being irradiated by any radiations after the first reading step; and a step of subtracting a signal value produced by averaging the second signal value and the third signal value with a signal processing unit processing the electric signal output from the reading circuit.
A program according to the present invention is a program stored in a storing medium for making a computer control an operation of a radiation image pickup apparatus provided with a detection unit in which a plurality of conversion elements converting a radiation into charges is two-dimensionally arranged, a driving circuit driving the detection unit, a reading circuit for reading an electric signal based on the charges from the detection unit, and a signal processing unit processing the electric signal output from the reading circuit, wherein the program makes the computer selectively execute a first reading procedure of reading a first signal value from the detection unit with the reading circuit by driving the detection unit irradiated by the radiation with the driving circuit, a second reading procedure of reading a second signal value from the detection unit with the reading circuit by driving the detection unit with the driving circuit without being irradiated by any radiations before the first reading procedure, and a third reading procedure of reading a third signal value from the detection unit with the reading circuit by driving the detection unit with the driving circuit without being irradiated by any radiations after the first reading procedure, and the program makes the computer execute a procedure of subtracting a signal value produced by averaging processing of the second signal value and the third signal value from the first signal value with the signal processing unit.
According to the present invention, because the first signal value is corrected using the signal value acquired by averaging the second signal value and the third signal value before and after acquiring the first signal value, an image lag can be remarkably reduced. Moreover, because the increase of the random noises accompanying the correction is also suppressed, it is possible to acquire a radiation image excellent in image quality and reliability. Furthermore, the faster the frame rate acquiring the first to the third signal values is set to be, the more the image lag can be reduced.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Hereinafter, an embodiment of the present invention is concretely described with reference to the attached drawings.
The radiation image pickup apparatus according to the present embodiment is provided with a detecting circuit unit 101, a driving circuit unit 103, an X-ray source (radiation generating apparatus) 109, a reading circuit unit 107, a signal processing circuit unit 108, and a control unit 105 performing the drive control of the X-ray source 109. Moreover, the radiation image pickup apparatus is configured so as to selectively freely set a moving image radiographing mode and a still image radiographing mode. In addition, although descriptions will be given to an X-ray image pickup apparatus in the present embodiment, the present invention is not limited to the X-ray image pickup apparatus. An α-ray, a β-ray, a γ-ray and the like are included in the category of the radiation.
The circuit configuration of a flat panel detector including the detecting circuit unit 101, the driving circuit unit 103 and the reading circuit unit 107 is shown in
In
The reading circuit unit 107 amplifies parallel signal outputs of the signal wiring M1-M3 in the detecting circuit unit 101, and performs the serial conversion of the amplified parallel signal outputs to output a converted signal. Reference marks RES1-RES3 denote switches for resetting the signal wiring M1-M3, and reference marks A1-A3 denote amplifiers amplifying the signals of the signal wiring M1-M3. Reference marks CL1-CL3 denote sample hold capacity, which temporarily stores the signals amplified by the amplifiers A1-A3, and reference marks Sn1-Sn3 denote switches for performing sample holds. Reference marks B1-B3 denote buffer amplifiers. Reference marks Sr1-Sr3 denote switches for performing serial conversions of parallel signals, and a reference mark SR2 denotes a shift register supplying pulses for performing serial conversions to the switches Sr1-Sr3. A reference numeral 104 denotes a buffer amplifier outputting a serially converted signal.
The operation of the radiation image pickup apparatus of the present embodiment is described.
First, a description is given to a converting period (radiation irradiating period). In a state in which all of the switching elements T1-1 to T3-13 are off, the X-ray source 109 is turned on in a way of a pulse. Then, the light according to a radiation dose is irradiated from the wavelength converter (not shown) to each of the photoelectric conversion elements S1-1 to S3-3. The charges corresponding to the quantity of light are converted by the photoelectric conversion elements S1-1 to S3-3, and the converted charges are accumulated in element capacity. When the wavelength converters are used, it is only necessary to use a member of guiding visible light according to the radiation dose to the photoelectric conversion element side, or to arrange light emitting bodies to the very vicinity of the photoelectric conversion elements. In addition, even after the radiation source has completed a radiation, the charges photoelectrically converted to the element capacity are held.
Next, a reading period is described. A reading operation is performed in the order of the first line of the photoelectric conversion elements S1-1 to S1-3, the second line of the photoelectric conversion elements S2-1 to S2-3 next, and then the third line of the photoelectric conversion elements S3-1 to S3-3. First, in order to read the photoelectric conversion elements S1-1 to S1-3 in the first line, a gate pulse is supplied to the gate wiring G1 connected to the switching elements (TFT) T1-1 to T1-3 in the first line from the driving circuit unit 103. Thereby, the switching elements T1-1 to T1-3 in the first line are turned to be their on states, and the charges of the photoelectric conversion elements S1-1 to S1-3 in the first line are transmitted to the signal wiring M1-M3. Reading capacity CM1-CM3 is added to the signal wiring M1-M3, and the charges are lead to be transmitted to the reading capacity CM1-CM3 through the switching elements T1-1 to T1-3. For example, the reading capacity CM1 added to the signal wiring M1 is the total sum (three pieces) of the interelectrode capacity (Cgs) between the gates and the sources of the switching elements T1-1 to T3-1 connected to the signal wiring M1. The charges transmitted to the signal wiring M1-M3 are amplified by the amplifiers A1-A3. And, the amplified charges are transmitted to the capacity CL1-CL3, and the transmitted charges are held at the timing of turning off an SMPL signal.
Next, pulses are applied to the switches Sr1, Sr2 and Sr3 in the same order from the shift register SR2, and thereby the signals held by the capacity CL1-CL3 are output from the amplifier 104 in the order of the capacity CL1, CL2 and CL3. That is, the analog signal outputs of the buffer amplifiers B1, B2 and B3 are output from the amplifier 104. From this, the sift register SR2 and the switches Sr1-Sr3 are inclusively called as an analog multiplexer. As a result, the signals according to the charges of the photoelectric conversion elements S1-1, S1-2 and S1-3 of one line are lead to be output one by one by the analog multiplexer. The reading operations of the photoelectric conversion elements S2-1 to S2-3 in the second line, and the reading operation of the photoelectric conversion elements S3-1 to S3-3 in the third line are performed similarly.
If the sample holds of the signals of the signal wiring M1-M3 in the capacity CL1-CL3 are performed with the SMPL signal of the first line, then the wiring M1-M3 is reset to the GND potential by a CRES signal, and a gate pulse on the gate wiring G2 can be applied after that. That is, while the serial conversion operation of the signals from the photoelectric conversion elements S1-1 to S1-3 in the first line are being performed, the charges of the photoelectric conversion elements S2-1 to S2-3 in the second line can be transferred to the signal wiring M1-M3 at the same time.
By the above operation, the charges according to the radiation, which have been generated in the photoelectric conversion elements S1-1 to S3-3 of all of the first to the third lines, can be output. And radiographing of a moving image is enabled by the repetition of such operations.
Here, a description will be given to the cross-sectional structure of an MIS type photoelectric conversion element and a switching element (thin film transistor (TFT)) included in the detecting circuit unit 101.
In such a structure of the conversion element, a radiation such as an X ray is converted into a visible light ray by the wavelength converter 110, and the visible light ray is converted into charges by the photoelectric conversion element 121. In addition, the conversion element may be configured so as to have a function in which the photoelectric conversion element 121 absorbs a radiation to convert the absorbed radiation into an electric signal directly without providing the wavelength converter 110. In this case, one kind selected from, for example, amorphous selenium, gallium arsenide, mercury iodide and lead iodide can be used as a material of the semiconductor layer of the conversion element. Moreover, the photoelectric conversion element 121 may be formed as a PIN type photoelectric conversion element.
Next, the signal processing circuit unit 108 is described.
Next, a reading operation (a drive method) in the present embodiment is described.
In
Next, the operation of the signal processing circuit unit 108 is described.
First, before a moving image radiographing start (the frames which are performing the operations in the periods “K” in
And a timing pulse is transmitted to a gate G13 using the shift register 5, and an acquired signal value is stored in the signal value memory 3. In the next frame, similarly to the above, a timing pulse is transmitted to a gate G12 using the shift registers 5 and 6 after the acquisition of the offset component, and the offset component output from the reading circuit unit 107 is stored in an offset memory 2. In this time, a signal value for correction is obtained by weighting the signal of the offset memory 1 and the signal of the offset memory 2 with 0.5 and by adding together the signals of the offset memories 1 and 2. And, an operation is conducted to subtract the signal valued for correction form the signal value of the signal value memory. While, in case of simple averaging processing, the offset signals 1 and 2 are added together, and subtracted from a carry digit (multiplied by 2). Thus, load of CPU would be reduced. And, as the weighting operation, a method with varying the weight for the offset signals 1 and 2 according to a frame rate of a photographing, or a method with a constant weight for the offset signals 1 and 2 fixed independently of the frame rate of the photographing may be selected. The subtracted value is set as a corrected value. And corrected values are acquired by the similar procedure hereinafter. Because the correction method performs the correction using the offset components before and after the acquisition of the signal of an X ray image, the method is called as a correction by offsets before and after.
In addition, as described above, the signal processing in the detecting circuit unit 101, the driving circuit unit (shift register) 103 and the reading circuit unit 107 while the signal processing is being performed is similar to the signal processing shown in
According to the present embodiment, as compared with the conventional radiation image pickup apparatus, image lags can be remarkably reduced. That is, in the conventional radiation image pickup apparatus, large image lags are generated immediately before and immediately after an input of a rectangular wave when the radiation image pickup apparatus receives a rectangular light signal (X ray signal). On the contrary, when a rectangular light signal is received while the signal processing shown in
Especially, if the image lag immediately after the completion of the light irradiation (radiation irradiation) is observed in
Furthermore, as shown in
The random noises included in the corrected value acquired according to the present embodiment are described. When it is supposed that a standard deviation of the random noises included in a signal value acquired by a normal reading operation is denoted by a letter σ1 and a standard deviation of the random noises included in an offset component output obtained by a reading operation for correction is denoted by a letter σ2. When the weight of offset component just before the photographing is a, and the weight of offset component just after the photographing is b, a standard deviation σt of the random noises included in the corrected values by offsets before and after according to the present embodiment is expressed by the following formula 2.
In particular, when the offset components just before and after the photographing are added together with weighting ratio 1:1 (0.5:0.5) , the standard deviation σt of the random noises is expressed by the following formula 3.
σ1=√{square root over (σ12+σ22/2)} (3)
Consequently, according to the present embodiment, it is also possible to reduce random noises.
Next, the application example of the radiation image pickup apparatus according to the embodiment of the present invention to a radiation image pickup system is described.
An X ray 6060 generated by an X ray tube 6050 (X-ray source 109) transmits a chest 6062 of a patient, or a subject, 6061, and enters an image sensor 6040 provided with a radiation image pickup apparatus according to the embodiment of the present invention therein. The image sensor 6040 includes the detecting circuit unit 101, the driving circuit unit 103, the reading circuit unit 107 and the signal processing circuit unit 108 of the above embodiment. The entered X ray includes the information of the inside of the body of the patient 6061. A scintillator (phosphor) emits light in correspondence with the incidence of the X-ray, and the photoelectric conversion element of the detecting circuit unit 101 performs the photoelectric conversion of the emitted light to acquire electric information. The image sensor 6040 outputs the information to an image processor 6070 as an electric signal (digital signal). The image processor 6070 as image processing means performs image processing to the received signal to output the processed signal to a display 6080 as the display means in a control room. A user observes the image displayed on the display 6080, and can acquire the information in the inside of the body of the patient 6061. In addition, the image processor 6070 also has a function of control means, and can switch the radiographing mode of a moving image/still image, or can perform the control of the X ray tube (radiation generating apparatus) 6050. That is, the image processor 6070 also functions as the control unit 105 in the above embodiment.
Moreover, the image processor 6070 can also transfer an electric signal output from the image sensor 6040 to a remote place through transmission processing means such as a telephone line 6090 and the like to display an image based on the electric signal on display means (display) 6081 located at another place such as a doctor room. Moreover, it is also possible to save the electric signal output from the image sensor 6040 into recording means such as an optical disk to perform a diagnosis by a doctor at a remote place using the recording means. Moreover, it is also possible to record the electric signal in a film 6110 with a film processor 6100 used as recording means.
In addition, the structure of the photoelectric conversion element of the present invention is not specially limited. For example, a photoelectric conversion element which is made of amorphous silicon as the main raw material and absorbs a visible light from a wavelength converter converting a radiation into a visible light to convert the absorbed visible light into an electric signal may be used. As such an element, for example, a PIN type photoelectric conversion element including a P layer, in which accepter impurities are doped, an I layers, which is an intrinsic semiconductor layer, and an N layer, in which donor impurities are doped, can be cited. Moreover, there can be cited an MIS type photoelectric conversion element including a metal thin film layer formed on a substrate, an insulating layer, which is formed on the metal thin film layer and is made of amorphous silicon nitride preventing the passage of electrons and holes, a semiconductor layer, which is made of hydrogenated amorphous silicon formed on the insulating layer, an N type impurity semiconductor layer, which is formed on the semiconductor layer and prevents the injection of holes, and a conductive layer formed on the impurity semiconductor layer. In the MIS type photoelectric conversion element, the conductive layer may be a transparent conductive layer, and the conductive layer may be formed in a part of the injection preventing layer. When these photoelectric conversion elements are used as the conversion element and a wavelength converter is needed, as the wavelength converter, for example, a phosphor made of Gd2O2S, Gd2O3 or CsI as the main components can be used. Furthermore, a device which includes amorphous selenium, gallium arsenide, lead iodide, or mercury iodide as a material of a semiconductor layer, and absorbs an irradiated radiation to convert the absorbed radiation into an electric signal directly without using any wavelength converters may be used as the conversion element.
Moreover, the structure of the reading circuit unit 107 is not particularly limited, either. For example, the structure including amplifying means for amplifying a signal read from the detecting circuit unit 101, accumulating means for accumulating the signal amplified by the amplifying means, and serial conversion means for performing the serial conversion of the signal accumulated by the accumulating means can be used.
In addition, the embodiment of the present invention can be realized by the execution of a program by a computer, for example. Moreover, the means for supplying a program to a computer, for example, a storing medium such as a CD-ROM, which has recorded such a program and can be read by a computer, or a transmitting medium such as the Internet, which transmits such a program, can be applied as an embodiment of the present invention. Moreover, the above program may be also applied as an embodiment of the present invention. The program, the storing medium, the transmitting medium and the program product are included in the category of the present invention.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims priority from Japanese Patent Application Nos. 2005-236773 filed on Aug. 17, 2005 and 2006-215855 filed on Aug. 8, 2006, which are hereby incorporated by reference herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-236773(PAT.) | Aug 2005 | JP | national |
| 2006-215855(PAT.) | Aug 2006 | JP | national |
