JP2010261828A - Detection method of defective pixel in two-dimensional array x-ray detector, and detection device of the defective pixel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a detection method of a defective pixel in a two-dimensional array X-ray detector, and a detection device of the defective pixel detecting the defective pixel having a growing property wherein the size becomes larger in a short time. <P>SOLUTION: This detection method of the defective pixel in the two-dimensional array X-ray detector includes: a dark current value measurement process for measuring a pixel value of each pixel with a flat panel detector not irradiated with an X-ray; and a determination process for determining as a defective pixel, a pixel wherein a dark current value of each pixel measured by the dark current value measurement is four and a half times or more as large as a pixel value of a normal pixel stored beforehand in a storage part. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a defective pixel detection method and a defective pixel detection apparatus in a two-dimensional array X-ray detector including a conversion film sensitive to X-rays and a two-dimensional array-shaped detection element that partitions pixels.

  As a two-dimensional array X-ray detector used in the X-ray imaging apparatus, for example, a flat panel detector (FPD) is known. This flat panel detector has a configuration in which a conversion film such as a-Se (amorphous selenium) is deposited on a substrate on which switching elements such as TFTs are arranged in a two-dimensional array (matrix). In this flat panel detector, when an X-ray image that has passed through the subject is projected onto the conversion film, a charge signal proportional to the density of the image is generated in the conversion film. This charge signal is collected by pixel electrodes arranged in a two-dimensional array and accumulated in a capacitance (capacitor). The electric charge accumulated in the capacitance is read with the operation of the switching element, and is transmitted as an electric signal to the image processing unit to perform image processing (see Patent Document 1).

JP 2008-301883 A

  In such a two-dimensional array X-ray detector using a conversion film such as a-Se, defective pixels may occur in the manufacturing process. Some of these defective pixels grow rapidly in size while using the two-dimensional array X-ray detector, and inhibit X-ray imaging in a short period of time. As described above, for a two-dimensional X-ray detector in which a defective pixel having a growth property that grows rapidly in a short period of time exists, this defective two-dimensional X-ray detection is impossible because the defective pixel makes correct X-ray imaging impossible. It is impossible to use the instrument in an X-ray imaging apparatus or the like.

  On the other hand, not all of the defective pixels are defective pixels having such growth potential that grows rapidly in such a short time. For defective pixels that do not have growth, it is possible to use a two-dimensional array X-ray detector having such defective pixels for X-ray imaging by registering the defect and complementing the pixel values.

  For this reason, it is necessary to determine whether a defective pixel grows rapidly or has no change in size. However, the number of defects and the defect size in a two-dimensional X-ray array detector are necessary. The present situation is that such a determination cannot be made even in consideration of the above.

  The present invention has been made to solve the above-described problem, and detects a defective pixel in a two-dimensional array X-ray detector capable of detecting a defective pixel having a growth property that increases in size in a short time. It is an object of the present invention to provide a method and a defective pixel detection apparatus.

  According to a first aspect of the present invention, there is provided a method for detecting a defective pixel in a two-dimensional array X-ray detector including a conversion film sensitive to X-rays and a two-dimensional array-shaped detection element that partitions pixels. A dark current value measuring step for measuring a pixel value of each pixel in a state in which irradiation is not performed, and a determination step for determining a defective pixel from the pixel value of each pixel measured in the dark current value measuring step. To do.

  According to a second aspect of the present invention, in the first aspect of the invention, in the determination step, the pixel value of each pixel measured in the dark current value measurement step is compared with the pixel value of a normal pixel. Thus, the defective pixel is determined.

  According to a third aspect of the present invention, in the second aspect of the present invention, in the determination step, the pixel value measured in the dark current value measurement step is not less than 4.5 times the pixel value of a normal image. Are determined as defective pixels.

  According to a fourth aspect of the present invention, in the method for detecting a defective pixel according to any one of the first to third aspects of the invention, the two-dimensional array X-ray detector is sensitive to X-rays and has an incident X-ray dose. A conversion film for outputting a corresponding charge signal, a plurality of pixel electrodes arranged in a matrix corresponding to the pixels on the surface of the conversion film, and a plurality of charge signals respectively connected to the pixel electrodes A storage capacitor; a switching element connected to the pixel electrode; a gate driver that sequentially turns on each switching element via a gate bus line when a signal is read; and a charge signal stored in each storage capacitor. A data accumulating unit that reads out via a line, and in the dark current value measuring step, each pixel is turned on sequentially with the switching elements turned on without irradiating the conversion film with X-ray To detect the charge signal.

  According to a fifth aspect of the present invention, there is provided an apparatus for detecting a defective pixel in a two-dimensional array X-ray detector including a conversion film sensitive to X-rays and a two-dimensional array-shaped detection element that partitions pixels. A dark current value measuring means for measuring a pixel value of each pixel in a state where the pixel is not irradiated, a storage means for storing a pixel value of a pixel to be a defective pixel, and a pixel value of each pixel measured by the dark current value measuring means A determination unit that determines whether the pixel is a defective pixel by comparing the pixel value of the pixel that is a defective pixel stored in the storage unit; And display means for displaying an error when it is determined that the pixel is a pixel.

  According to a sixth aspect of the present invention, in the fifth aspect of the present invention, the storage unit stores a pixel value 4.5 times the pixel value of a normal image as a pixel value of a pixel that becomes a defective pixel.

  According to the first aspect of the present invention, it is possible to detect a defective pixel having a growth property that increases in size in a short time while distinguishing it from other defective pixels.

  According to the invention according to claim 2, claim 3, claim 5 and claim 6, since the determination is made by comparison with a normal pixel value, it has a growth property that its size increases in a short time. It is possible to detect a defective pixel more accurately by distinguishing it from other defective pixels.

  According to the fourth aspect of the present invention, it is possible to detect a defective pixel having a growth property that increases in size in a short time, which occurs in the X-ray conversion film, separately from other defective pixels. For this reason, a defective pixel having a growth property that is specifically generated based on the characteristics of an X-ray conversion film that is sensitive to X-rays and outputs a charge signal corresponding to an incident X-ray dose is detected separately from a general defective pixel. It becomes possible.

1 is a schematic diagram of an X-ray imaging apparatus to which the present invention is applied. It is the equivalent circuit which looked at the flat panel detector 4 from the side. It is an equivalent circuit in plan view of the flat panel detector 4. It is a graph explaining the offset and gain of the sensitivity of the flat panel detector. It is a flowchart which shows the detection method of a missing pixel.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, the configuration of an X-ray imaging apparatus to which the flat panel detector 4 as a two-dimensional array X-ray detector according to the present invention is applied will be described. FIG. 1 is a schematic view of an X-ray imaging apparatus to which a flat panel detector 4 according to the present invention is applied.

  The X-ray imaging apparatus includes a table 2 on which a subject 1 as a subject is placed, an X-ray tube 3, a flat panel detector 4, an A / D converter 5, an image processing unit 61, and a calculation unit. 62, a storage unit 63, an input unit 64 such as a keyboard, a display unit 65 such as a CRT, and an X-ray tube control unit 7 that controls a tube voltage applied to the X-ray tube 3. Is provided.

  This X-ray imaging apparatus emits X-rays from an X-ray tube 3 toward a subject 1 on a table 2, detects X-rays passing through the subject 1 by a flat panel detector 4, and an image processing unit The X-ray detected in 61 is subjected to image processing, and an X-ray fluoroscopic image is displayed on the display unit 65 using a video signal of the image-processed X-ray.

  Next, the configuration of the flat panel detector 4 will be described. FIG. 2 is an equivalent circuit when the flat panel detector 4 is viewed from the side. FIG. 3 is an equivalent circuit of the flat panel detector 4 in plan view.

  As shown in these drawings, the flat panel detector 4 includes a glass substrate 41, a TFT (thin film transistor) 42 formed on the glass substrate 41, and a conversion film such as a-Se deposited on the TFT. 43 and a common electrode 44 disposed on the conversion film 43.

  The TFT is provided with pixel electrodes 45 serving as charge collection electrodes in a matrix arrangement in the vertical and horizontal directions, that is, in a two-dimensional array. For example, 1024 × 1024 pixel electrodes 45 are arranged in the matrix direction. 2 and 3 schematically show a case where 3 × 3 are arranged in the matrix direction. A switching element 46 and a capacitance (capacitor) 47 are connected to each pixel electrode 45.

  Each pixel electrode 45 is connected to the source S of each switching element 46. A plurality of gate bus lines 52 are connected to the gate driver 51 shown in FIG. 3, and these gate bus lines 52 are connected to the gate G of the switching element 46. On the other hand, as shown in FIG. 3, a plurality of data bus lines 55 are connected via an amplifier 54 to a multiplexer 53 that collects charge signals and outputs them to one. Are connected to the drain D of each switching element 46.

  In the flat panel detector 4, when an X-ray image that has passed through the subject 1 is projected onto the conversion film 43, a charge signal (carrier) proportional to the density of the image is generated in the conversion film 43. This charge signal is collected by the pixel electrodes 45 arranged in a two-dimensional array and accumulated in the capacitance 47. Then, the gate G of each switching element 46 is turned on by applying a voltage to the gate bus line 52 by the gate driver 51 in a state where the bias voltage is applied to the common electrode 44. As a result, the charge signal accumulated in the capacitance 47 is read out to the data bus line 55 via the source S and drain D in the switching element 46. The charge signals read to the data bus lines 55 are amplified by the amplifiers 54 and are combined into one charge signal by the multiplexer 53 and output. This charge signal is digitized by the A / D converter 5 and output to the control unit 6 shown in FIG. 1 as an X-ray detection signal.

  In the flat panel detector 4 having such a configuration, a defective pixel may occur in the manufacturing process due to the characteristics of the conversion film 42 made of a-Se or the like. Some of these defective pixels grow rapidly in size while the flat panel detector 4 is used. In the present invention, it is possible to determine whether or not the growth defect is based on the dark current value obtained during calibration for measuring the offset and gain of the sensitivity of the flat panel detector 4. It is based on what the inventor has found.

  FIG. 4 is a graph illustrating the sensitivity offset and gain of the flat panel detector 4 measured during calibration.

  In this figure, the horizontal axis indicates the X-ray dose irradiated to the flat panel detector 4, and the vertical axis indicates the pixel value of each pixel. In order to create this graph, first, the pixel value of each pixel of the flat panel detector 4 when the flat panel detector 4 called a dark current value is not irradiated with X-rays is measured. Next, the pixel value of each pixel when the flat panel detector 4 is uniformly irradiated with X-rays is measured. And the graph shown in FIG. 4 is obtained from those pixel values.

  In FIG. 4, a value indicated by an arrow is a dark current value that is a pixel value when the flat panel detector 4 is not irradiated with X-rays, and this represents a sensitivity offset in the flat panel detector 4. Further, the slope of the graph in FIG. 4 represents the gain of sensitivity in the flat panel detector 4. It has been found that whether or not each pixel is a defective pixel having a growth property can be determined from this dark current value.

  Next, a defective pixel detection method according to the present invention for determining whether or not each pixel in the flat panel detector 4 is a defective pixel having growth properties will be described. FIG. 5 is a flowchart showing a method for detecting a defective pixel.

  In order to detect a defective pixel, first, the dark current value of each pixel is measured (step S1). At this time, by sequentially turning on the switching elements 46 in a state where the conversion film 43 in the flat panel detector 4 is not irradiated with X-rays, the charge signal of each pixel of the flat panel detector 4 is detected as a pixel value. The pixel value of each pixel at this time is stored in the storage unit 63 shown in FIG. 1 as a dark current value. Note that the pixel values of normal pixels are also stored in advance in the storage unit 63 illustrated in FIG.

  Next, for each pixel in the flat panel detector 4, a dark current value that is a pixel value of each pixel in a state where X-rays are not irradiated is compared with a pixel value of a normal pixel. Then, a pixel in which the dark current value of each pixel is 4.5 times or more that of a normal pixel is determined as a defective pixel (step S2). When such a defective pixel is found, the pixel value and position of the defective pixel are stored in the storage unit 63. This comparison and determination is executed by the calculation unit 62 in the control unit 6.

  Here, a pixel whose dark current value is 4.5 times or more the pixel value of a normal pixel is determined as a defective pixel while such a pixel is large while the flat panel detector 4 is used. This is because it has been experimentally confirmed that the pixel grows rapidly and is a defective pixel that inhibits X-ray imaging in a short period of time.

  Table 1 below shows the defective pixels of five flat panel detectors and dark currents of the pixels in which defective pixels whose sizes grew in a short period of time were found as a result of inspection on a large number of flat panel detectors. It is a table | surface which shows a value. In this table, the panel number indicates the number of the flat panel detector. This table shows that one defect was found for each of the flat panel detectors with panel numbers 1, 2, 3, and 5, and five defects were found for the flat panel detector with panel number 4. It is shown that. And each dark current value has shown the value measured beforehand about those defective pixels. As is apparent from this table, for the defective pixels whose size grows rapidly in a short time, the dark current value measured in advance is 9.2 picoamperes or more.

  According to the inventor's research, a pixel whose dark current value is 4.5 times or more of the pixel value of a normal pixel may become a defective pixel having a growth property whose size rapidly increases in a short time. Has been confirmed to be high. It is extremely unlikely that a pixel having a dark current value smaller than 4.5 times the pixel value of a normal pixel will be a defective pixel having growth properties. A pixel having a dark current value smaller than twice the pixel value of a normal pixel is unlikely to be a defective pixel having growth properties. For this reason, it is preferable to determine a pixel whose dark current value is 4.5 times or more the pixel value of a normal pixel as a defective pixel, and a pixel whose dark current value is twice or more the pixel value of a normal pixel. More preferably, it is determined as a defective pixel.

  Here, the dark current value of a normal pixel of a general flat panel detector 4 is approximately 2 picoamperes. For this reason, the determination of the defective pixel is preferably based on whether the dark current value is 9 picoamperes or more, and more safely, based on whether the dark current value is 4 picoamperes or more. It is more preferable. The dark current value serving as a reference for comparison at this time is input in advance by the operator using the input unit 64 and stored in the storage unit 63.

  The above comparison and determination are executed for all pixels. When the comparison and determination for all the pixels are completed (step S3), and no defective pixel is found (step S4), it is determined that the flat panel detector 4 is good, and the process ends. On the other hand, when a defective pixel is found (step S4), it is determined that the flat panel detector 4 is a defective product. In this case, the control unit 6 displays an error on the display unit 65 (step S5) and ends the process.

  In the above-described embodiment, the dark current value that becomes the reference for comparison when determining the defective pixel is previously input by the operator using the input unit 64 and stored based on the dark current value of the normal pixel. Stored in the unit 63. However, an average value of dark current values of all the pixels of the flat panel detector 4 may be used as a dark current value of a normal pixel.

  That is, even if there is a growing defective pixel, since the defective pixel is only a part of all the pixels of the flat panel detector 4, the average dark current value of all the pixels of the flat panel detector 4 is: This is very close to the dark current value of a normal pixel. For this reason, the average value of the dark current values of all the pixels measured in the dark current value measuring step (step S1) is calculated by the calculation unit 62 of the control unit 6, and this is stored in the storage unit 63 as the dark current value of the normal pixels. Remember. In the pixel value comparison / determination step (step S2), pixel values may be compared and missing pixels may be determined using the average value stored as the dark current value of normal pixels.

  In the above-described embodiment, the case where the present invention is applied to the flat panel detector 4 used in the X-ray imaging apparatus has been described. However, other two-dimensional array X-ray detection having a conversion film sensitive to X-rays. The present invention can be applied to a vessel.

DESCRIPTION OF SYMBOLS 1 Subject 2 Table 3 X-ray tube 4 Flat panel detector 6 Control part 7 X-ray tube control part 41 Glass substrate 42 TFT
43 Conversion Film 44 Common Electrode 45 Pixel Electrode 46 Switching Element 47 Capacitance 51 Gate Driver 52 Gate Bus Line 53 Multiplexer 54 Amplifier 55 Data Bus Line 61 Image Processing Unit 62 Calculation Unit 63 Storage Unit 64 Input Unit 65 Display Unit

Claims (6)

In a method for detecting a defective pixel in a two-dimensional array X-ray detector comprising a conversion film sensitive to X-rays and a two-dimensional array-shaped detection element that partitions pixels,
A dark current value measuring step of measuring a pixel value of each pixel in a state where X-rays are not irradiated;
A determination step of determining a defective pixel from a pixel value of each pixel measured in the dark current value measurement step;
A method of detecting a defective pixel in a two-dimensional array X-ray detector. The method for detecting a defective pixel in the two-dimensional array X-ray detector according to claim 1,
In the determination step, the pixel value of each pixel measured in the dark current value measurement step is compared with the pixel value of a normal pixel, thereby determining the defective pixel in the two-dimensional array X-ray detector that determines the defective pixel. Detection method. The method for detecting a defective pixel in the two-dimensional array X-ray detector according to claim 2,
In the determination step, a defective pixel is detected by a two-dimensional array X-ray detector that determines a pixel whose pixel value measured in the dark current value measurement step is 4.5 times or more that of a normal image as a defective pixel. Method. In the detection method of the defect pixel in the two-dimensional array X-ray detector in any one of Claims 1 thru | or 3,
The two-dimensional array X-ray detector includes a conversion film sensitive to X-rays and outputting a charge signal corresponding to an incident X-ray dose, and a plurality of pixels arranged in a matrix corresponding to the pixels on the surface of the conversion film Electrodes, a plurality of storage capacitors for storing charge signals respectively connected to the pixel electrodes, switching elements connected to the pixel electrodes, and switching elements are sequentially turned on via a gate bus line when signals are read out. A gate driver, and a data integration unit that reads out a charge signal stored in each storage capacitor via a data bus line,
In the dark current value measurement step, a defective pixel detection method in a two-dimensional array X-ray detector that detects a charge signal of each pixel by sequentially turning on the switching elements without irradiating the conversion film with X-rays. In a defect pixel detection apparatus in a two-dimensional array X-ray detector comprising a conversion film sensitive to X-rays and a two-dimensional array-shaped detection element that partitions pixels,
Dark current value measuring means for measuring a pixel value of each pixel in a state in which X-rays are not irradiated;
Storage means for storing a pixel value of a pixel to be a defective pixel;
By comparing the pixel value of each pixel measured by the dark current value measuring unit with the pixel value of the pixel that becomes the defective pixel stored in the storage unit, it is determined whether or not the pixel is a defective pixel. A determination means;
Display means for displaying an error when the determination means determines that any of the pixels is a defective pixel;
An apparatus for detecting a defective pixel in a two-dimensional array X-ray detector. The defect pixel detection apparatus in the two-dimensional array X-ray detector according to claim 5,
The storage means is a defective pixel detection device in a two-dimensional array X-ray detector that stores a pixel value 4.5 times the pixel value of a normal image as a pixel value of a pixel that becomes a defective pixel.

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