JP5181882B2 - Radiation imaging device - Google Patents

Description

  The present invention relates to a radiation imaging apparatus used in the medical field, industrial fields such as non-destructive inspection, RI (Radio isotope) inspection, and optical inspection, and in the nuclear power field, and in particular, a pixel of a flat panel type radiation detector. The present invention relates to a radiation imaging apparatus that performs calibration (calibration) and a calibration method for the radiation imaging apparatus.

  A flat panel X-ray detector (hereinafter referred to as FPD) used in a radiation imaging apparatus is configured by laminating a sensitive film sensitive to radiation (X-rays) on an active matrix substrate. When the radiation is incident on the sensitive film, the radiation is converted into charges, and the charges are accumulated in the capacitors Ca arranged in a two-dimensional array. The accumulated charge is read by turning on the switching element (TFT element) of the active matrix substrate, and is converted from a charge signal to a voltage signal on the way, and then sent to the image processing unit. However, even if the same radiation dose is incident, the amount of charge converted for each pixel (detection element) varies, and as a result, the amount of charge accumulated in the capacitor Ca also varies. As a result, even if the same radiation dose is incident, the charge signal converted for each pixel also varies, and a transmitted X-ray image corresponding to the correct radiation dose cannot be displayed. In order to eliminate such variation, calibration (calibration) is performed to adjust the amplification (gain) of the charge signal for each pixel to align the output side (see Patent Document 1). In this calibration, the gain of the amplifier (amplifier) may be directly adjusted, but can also be adjusted by the image processing unit. Since the image processing unit processes the voltage signal for each pixel as image data, the amplification can be easily adjusted.

  When calibration is performed by the image processing unit, calibration irradiation data is usually created by averaging a plurality of pieces of image data obtained by FPD, and is sent to each pixel based on the calibration irradiation data. The gain of the received image data is adjusted. The gain value for each pixel is referred to as calibration data.

  In addition, although an amorphous selenium (a-Se) semiconductor film is used as a sensitive film sensitive to radiation in FPD, a time-dependent change occurs in the a-Se semiconductor film such that a-Se crystallizes with time. Due to this change over time, a portion that is not sensitive to radiation is generated in the a-Se semiconductor film, and the X-ray detection value (pixel value) of the pixel becomes extremely small by not being sensitive to radiation. On the other hand, the conductivity of the a-Se semiconductor film increases, and the pixel value may become extremely large due to the increased dark current. In this way, pixels with extremely small pixel values and pixels with extremely large pixel values are called FPD defective pixels.

As described above, since crystallization of a-Se and increase in dark current occur with time, a defective pixel is generated at a place where there is no defective pixel in the FPD at the beginning. May spread to the periphery and be deficient for two or three pixels. Accordingly, it is necessary to perform calibration of the FPD periodically, and it is desirable that the calibration data (gain value) is always updated to the latest numerical value.
JP 2005-204983 A

  However, when a foreign object is present on the surface of the FPD at the time of calibration, the reflection due to the foreign object affects the calibration irradiation data, and accurate calibration data cannot be acquired. In other words, even if a foreign object is reflected in the calibration irradiation data, this foreign object is not noticed, and new calibration data is created. An accurate transmission X-ray image could not be constructed. Furthermore, since the calibration data is updated every time calibration is performed, image degradation occurs every time X-ray imaging is performed using the calibration data.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a radiation imaging apparatus capable of detecting foreign matter on a radiation detector.

That is, according to the first aspect of the present invention, in the radiation imaging apparatus comprising the radiation irradiating means for irradiating radiation and the radiation detecting means for detecting the radiation irradiated by the radiation irradiating means, the radiation detecting means A first arithmetic unit that calculates calibration irradiation data by taking an average of a plurality of detected image data, and averaging the calibration irradiation data and the calibration irradiation data when the previous calibration was performed . The foreign matter on the radiation irradiating means is detected by comparing the calibration calculating data with the second arithmetic unit for calculating the calibration judgment data in the above and the threshold value for discriminating the foreign and defective pixels of the radiation detecting means with the calibration judgment data. And a foreign matter detector.

According to the above configuration, when there is a foreign substance that absorbs radiation between the radiation irradiating means and the radiation detecting means, the detected value of the radiation is lower than the other pixels in the pixel that is the shadow of the foreign substance. A pixel that is a shadow of the foreign object is detected in comparison with the threshold value. This makes it possible to detect foreign matter. In addition, by using calibration determination data, which is an average of calibration irradiation data and past calibration irradiation data, as a comparison target with a threshold value, it is possible to observe a temporal change of a defective pixel and a foreign pixel. In addition, since the calibration irradiation data at the time of the previous calibration and the calibration irradiation data at the time of the current calibration can be compared, the radiation detector of the last time the calibration was performed The state can be compared with the current radiation detector state.

  According to a second aspect of the present invention, in the radiation imaging apparatus according to the first aspect, the threshold value is a predetermined value based on an empirical rule.

  According to the above configuration, since the threshold can be set by the operator, the detection accuracy of the foreign matter can be adjusted depending on the imaging target.

  According to a third aspect of the present invention, the radiation imaging apparatus according to the first aspect further includes a threshold value calculation unit that calculates the threshold value from a distribution function of the calibration irradiation data every time calibration is performed. It is characterized by.

  According to the above configuration, it is possible to detect a foreign object by calculating an optimum threshold every time calibration is performed.

  According to a fourth aspect of the present invention, the radiation imaging apparatus according to any one of the first to third aspects further comprises a warning unit that warns the presence of the foreign matter detected by the foreign matter detection unit. Features.

  According to the above configuration, the presence of the foreign matter is warned by the warning means, so that the operator can know the presence of the foreign matter. Thus, the operator can remove the foreign matter by confirming the radiation detection means.

  According to a fifth aspect of the present invention, in the radiation imaging apparatus according to any one of the first to fourth aspects, pixel complementation that supplements a radiation detection value of a pixel that is prevented from being irradiated with radiation by the foreign matter. It comprises a part.

  According to the above configuration, even when there is a foreign object that cannot be removed, it is possible to complement the radiation detection value of the pixel that is blocked by the radiation due to the shadow of the foreign object. can do.

  According to the radiation imaging apparatus according to the present invention, it is possible to provide a radiation imaging apparatus that can detect a foreign object on the radiation detector.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of the X-ray diagnostic apparatus according to the embodiment, FIG. 2 is an equivalent circuit of the FPD used in the X-ray diagnostic apparatus as viewed from the side, and FIG. 3 is an equivalent circuit of the FPD as viewed from above. FIG. 4 is a block diagram of the image processing unit according to the embodiment. In the present embodiment, an X-ray diagnostic apparatus will be described as an example of a radiation imaging apparatus.

<X-ray diagnostic equipment>
As shown in FIG. 1, the X-ray diagnostic apparatus according to the present embodiment includes a top plate 1 on which a subject is placed, an X-ray tube 2 that irradiates the subject with X-rays, the subject and the celestial body. An FPD 3 for detecting X-rays transmitted through the plate 1 and a top plate control unit 4 for controlling the elevation and horizontal movement of the top plate 1 are provided. The X-ray tube 2 corresponds to the radiation irradiation means in this invention, and the FPD 3 corresponds to the radiation detection means in this invention.

  The X-ray diagnostic apparatus also includes an FPD control unit 5 that controls the position of the FPD 3, an X-ray tube control unit 7 that has a high voltage generation unit 6 that generates the tube voltage and tube current of the X-ray tube 2, and the FPD 3 An image processing unit 8 that performs calibration and various image processing based on an X-ray detection signal that is a voltage signal, a controller 9 that supervises these components, an input unit 10 that an operator performs input settings, An X-ray diagnostic operation screen and a monitor 11 for displaying an image-processed X-ray transmission image and the like are provided. The monitor 11 corresponds to warning means in the present invention.

  The top board control unit 4 moves the top board 1 up and down and horizontally moves it to set the subject to a desired position, performs imaging while moving horizontally, or horizontally moves after the imaging is finished and retreats from the imaging position. And so on. The FPD control unit 5 performs control related to horizontally moving the FPD 3 or rotating the FPD 3 around the body axis of the subject. The high voltage generation unit 6 generates a tube voltage and a tube current for irradiating X-rays and applies them to the X-ray tube 2. The X-ray tube control unit 7 moves the X-ray tube 2 horizontally, Control related to scanning that rotates around the axis of the body axis, control of the setting of the irradiation field of a collimator (not shown) in the X-ray tube 2 and the like are performed. When scanning the X-ray tube 2 or the FPD 3, the X-ray tube 2 and the FPD 3 move while facing each other so that the FPD 3 can detect the X-rays emitted from the X-ray tube 2.

  The controller 9 includes a central processing unit (CPU). The input unit 10 includes a pointing device represented by a mouse, keyboard, joystick, trackball, touch panel, and the like.

<Flat panel X-ray detector>
As shown in FIG. 2, the FPD 3 includes an insulating substrate 12 and a thin film transistor (hereinafter referred to as TFT) 13 formed on the insulating substrate 12. As shown in FIGS. 2 and 3, the TFT 13 is formed by arranging a large number (for example, 1024 × 1024) in a vertical and horizontal two-dimensional matrix, and the TFT 13 is switched for each pixel electrode 14. The elements are separated from each other. That is, the FPD 3 is also a two-dimensional array radiation detector.

  As shown in FIG. 2, an X-ray sensitive semiconductor 15 is stacked on the pixel electrode 14, and the pixel electrode 14 is connected to the source S of the TFT 13 as shown in FIGS. 2 and 3. A plurality of gate bus lines 17 are connected from the gate driver 16, and each gate bus line 17 is connected to the gate G of the TFT 13. On the other hand, as shown in FIG. 3, a plurality of data bus lines 30 are connected via an amplifier 19 to a multiplexer 18 that collects charge signals and outputs them to one. Thus, each data bus line 20 is connected to the drain D of the TFT 13.

  With the bias voltage Va applied to the common electrode 21, the gate of the TFT 13 is turned on by applying the voltage of the gate bus line 17 (or 0 V), and the pixel electrode 14 is incident on the detection surface side of the X-ray The charge signal (carrier) converted through the X-ray sensitive semiconductor 15 is read out to the data bus line 20 through the source S and drain D of the TFT 13. Until the TFT 13 is turned on, the charge signal is provisionally accumulated and stored in the capacitor Ca. The charge signal read out to each data bus line 20 is converted from a charge signal to a voltage signal by an amplifier 19 and amplified, and then output to a single voltage signal by a multiplexer 18. The output voltage signal is digitized by the A / D converter 22 and output as an X-ray detection signal.

  At this time, if a portion not sensitive to X-rays occurs in the X-ray sensitive semiconductor 15, the number of charge signals stored in the capacitor Ca is smaller than the charge signals stored in other capacitors Ca. Further, when the dark current increases in the X-ray sensitive semiconductor 15, the detected charge signal becomes extremely large. A pixel in any one of these states must be detected as a defective pixel because it cannot accurately detect X-rays. Thus, detecting the defective pixel of the FPD 3 is one of the reasons for performing calibration.

<Image processing unit>
As shown in FIG. 4, the image processing unit 8 stores a voltage signal (image data) sent from the FPD 3, and adds and averages a plurality of pieces of image data stored in the first memory unit 31. Then, the first calculation unit 34 for calculating the calibration irradiation data, the calibration irradiation data and the calibration reference data stored in advance in the second memory unit 32 are added and averaged to calculate calibration determination data. A second computing unit 35, a foreign matter detection unit 36 for detecting foreign matter pixels from the calibration determination data, a pixel complementing unit 37 for complementing the detected foreign matter pixels, and an amplification for determining an amplification factor (gain value) for each pixel When the image of the subject is imaged with the rate calculation unit 38, the image data obtained by transmitting through the subject is corrected with the gain value, and the image constituting the X-ray transmission image It consists of correcting section 39.. The X-ray transmission image formed by the image correction unit 39 is displayed on the monitor 11 via the controller 9. Here, the foreign matter pixel means a pixel whose X-ray detection value is lowered due to the foreign matter present on the FPD 3. The first memory unit and the second memory unit constitute a memory unit 33.

  Next, a series of calibration control sequences in the present embodiment will be described with reference to the flowchart of FIG.

(Step S1) Acquisition of calibration irradiation data X-rays are irradiated from the X-ray tube 2 in a state where nothing is placed on the top board 1, and image data is acquired by the FPD 3. At this time, it is desirable to acquire about several tens of pieces of image data. This image data is sent from the FPD 3 to the first memory unit 31 in the image processing unit 8 and stored therein. The image data stored in the first memory unit 31 is sent to the first calculation unit and subjected to an averaging process to calculate calibration irradiation data.

(Step S2) Calibration Determination Data Calculation The calibration irradiation data calculated by the first calculation unit 34 and the calibration reference data stored in advance in the second memory unit are added and averaged by the second calculation unit 35. Calibration determination data is calculated by processing. Here, if the difference process of subtracting the calibration reference data from the calibration irradiation data is performed, the calculated image data cannot be distinguished from the defective pixel, the normal pixel, and the foreign pixel. Is preferred.

  For example, as shown in FIG. 6A, the pixel value of a defective pixel having an extremely high X-ray detection value is set to 100, the pixel value of a defective pixel having an extremely low X-ray detection value is set to 1, and a foreign pixel The pixel value of the normal pixel is 40, and the pixel value of the normal pixel is 60. Here, since the calibration reference data stored in the second memory unit is calibration irradiation data when the previous calibration is performed, the calibration reference data includes an extremely low pixel value of the missing pixel. In addition, an extremely high pixel value and a pixel value of a foreign substance pixel due to a foreign substance that cannot be removed are included.

  When the subtraction process is performed by the second calculation unit 35, in the calibration irradiation data and the calibration reference data, the defective pixel and the foreign pixel that cannot be removed are often the same pixel on the two-dimensional matrix. The subtraction of the pixel value between pixels, the subtraction of the pixel value between foreign pixels, and the subtraction of the pixel value between normal pixels all result in zero. As a result, the next foreign object detection unit 36 cannot determine whether a pixel having a zero pixel value is a defective pixel, a foreign object pixel, or a normal pixel. Therefore, if the averaging process is performed, the above problem can be avoided.

(Step S3) Foreign object detection As shown in FIGS. 6A and 6B, normal pixels, foreign object pixels, and defective pixels are determined from the calibration determination data calculated by the second arithmetic unit 35, as shown in FIGS. Is detected. 6A and 6B, a threshold A is a threshold for discriminating a defective pixel whose X-ray detection value is extremely high due to dark current, and a threshold C is an X-ray detection value obtained by crystallization of a-Se. Is a threshold for discriminating defective pixels that are extremely low. The calibration determination data is compared with the threshold A and threshold C, and a pixel having a pixel value higher than the threshold A and a pixel having a pixel value lower than the threshold C are determined as defective pixels. The value of the threshold A is 70 as an example, and the value of the threshold C is 35, for example.

  Next, the normal pixel and the foreign pixel are discriminated by comparing the calibration determination data in which the defective pixel is determined with the threshold B. If there is a foreign object on the surface of the FPD 3, a foreign object pixel that is a shadow of the foreign object is generated when radiation is irradiated, and the pixel value of the foreign object pixel is lower than a pixel that is not shadowed by another foreign object. . Therefore, a pixel having a pixel value lower than a predetermined threshold B is determined as a foreign pixel. In this embodiment, the threshold value B is 55. Note that the values of the thresholds A, B, and C may be set based on empirical rules, and may be set appropriately as appropriate, so that they are not limited to the above values.

  FIG. 6B shows calibration determination data in the case where a pixel value as a defective pixel or a foreign pixel is detected at the time of the current calibration, which was a normal pixel when the previous calibration was performed. ing. That is, the pixel value 80 and the pixel value 30.5 obtained by averaging the pixel value 60 that is the normal pixel of the calibration reference data and the pixel value 100 and the pixel value 1 that are the defective pixels of the calibration irradiation data are set to the normal pixel → defect. Shown as pixels. Similarly, a pixel value 50 obtained by averaging the pixel value 60 that is a normal pixel of the calibration reference data and the pixel value 40 that is a foreign pixel of the calibration irradiation data is shown as normal pixel → foreign pixel. As described above, even when a new defective pixel and foreign pixel are generated during the current calibration, the threshold A, the threshold B, and the threshold C can be distinguished. Further, if the calibration reference data and the calibration determination data are displayed on the monitor 11 via the controller 4, newly generated defective pixels can be confirmed on the monitor 11 as compared with the previous calibration, so that the operator The change in the state of the pixel of the FPD 3 can be notified.

(Step S <b> 4) Foreign object removal When a foreign pixel is detected, a foreign object detection signal is sent to the controller 4. Based on the foreign matter detection signal, the controller 4 displays a foreign matter detection warning on the screen on the monitor 11 to notify the operator of the presence of the foreign matter. At this time, the monitor 11 displays where the foreign matter is attached on the surface of the FPD 3, so that the operator can confirm whether or not the foreign matter is attached on the surface of the FPD 3. If there is a foreign object, it is removed and the calibration irradiation data is obtained again.

  If there is no foreign matter or if the foreign matter cannot be removed, the calibration is continued as it is. Examples of the case where the foreign matter cannot be removed include a case where there is a foreign matter between the outer cover of the FPD 3 housing and the common electrode 21, and a case where the foreign matter cannot be removed without disassembling the FPD 3. When the operator determines that the foreign matter cannot be removed, a calibration continuation signal is sent from the input unit 10 to the image processing unit 8 via the controller 9. Thus, the calibration determination data is sent to the pixel complementing unit 36, and the calibration irradiation data is sent to the second memory unit to be replaced with the calibration reference data. When there is no foreign object, the above processing is automatically performed. In this way, the calibration reference data is updated each time calibration is performed.

(Step S <b> 5) Pixel Supplementation The pixel complementation unit 37 supplements the foreign matter pixel and the defective pixel detected by the foreign matter detection unit. Specifically, median filter processing that replaces foreign pixels and missing pixels with median pixels from a plurality of normal pixels adjacent to foreign pixels and missing pixels, and appropriate complementation such as weighting based on adjacent normal pixels Processing is performed to complement foreign and missing pixels.

(Step S6) Amplification factor calculation Amplification factor calculation is performed on the amplification factor (gain value) for each pixel detected so that each X-ray detection value is aligned based on the calibration determination data complemented with the foreign pixel and the defective pixel. Calculated by the unit 38. As described above, a gain value for each pixel, that is, calibration data can be created.

  Based on the calibration data, the image correction unit 39 corrects the pixel value (radiation detection value) for each pixel by imaging the subject and amplifying the image data stored in the first memory unit 31. Thereby, an optimal transmission X-ray image can be obtained.

  According to the radiation diagnostic apparatus according to the present embodiment, calibration that is not affected by foreign matter is performed by accurately detecting foreign matter attached on the radiation detector and removing the foreign matter or complementing pixels that are shadowed by the foreign matter. Data can be created.

  In the embodiment described above, the foreign matter attached on the FPD 3 is detected. However, even if the foreign matter is not directly attached on the FPD 3 or the foreign matter is present between the X-ray tube 2 and the FPD 3, the calibration is performed. Sometimes a decrease in image data is detected, so that foreign matter can be detected. Further, not only a foreign substance but also, for example, even when the viewing angle of the X-ray tube 2 does not coincide with the FPD 3, a decrease in image data is detected during calibration, so that a viewing angle shift of the X-ray tube 2 is detected. Can do.

  The present invention is not limited to the above-described embodiment, and can be modified as follows.

  (1) In the above-described embodiment, the calibration determination data is an addition average of the calibration irradiation data and the calibration reference data, but may be a geometric average. Alternatively, a combination of addition average and geometric average may be used. The thresholds A, B, and C in this case may be changed as appropriate.

  (2) In the above-described embodiment, the threshold value for discriminating between the normal pixel and the foreign pixel is a fixed value determined in advance, but may be a threshold value calculated every time calibration is performed. In this case, as shown in FIG. 7, a threshold value calculation unit 40 that calculates a threshold value from calibration irradiation data may be provided. The threshold value calculation method may be obtained from, for example, a distribution function of pixel values of calibration irradiation data, or intermediate values of pixel values of typical normal pixels, pixel values of defective pixels, and pixel values of foreign matter pixels. The value may be calculated as a threshold value.

  An example of obtaining the threshold value from the distribution function of the pixel values of the calibration irradiation data will be described with reference to FIG. FIG. 8 is a semilogarithmic graph showing the relationship between the pixel value of the calibration irradiation data and the number of pixels. The minimum point of this distribution function is calculated, and the higher pixel value is set as the threshold value A, and the lower pixel value is set as the threshold value C. Further, a range d occupying 95% of the number of pixels is obtained from the pixel value of the maximum point, and the pixel value which is the lower limit of this range is set as the threshold value B. The method for calculating the thresholds A, B, and C is not limited to this method, and other statistical methods may be used.

  In addition, a case will be described in which an intermediate value between a pixel value of a typical normal pixel, a pixel value of a defective pixel, and a pixel value of a foreign object pixel is calculated as a threshold value. For example, when the threshold value C is set lower than the pixel value 30.5 obtained by averaging the pixel value 60 of the normal pixel and the pixel value 1 of the defective pixel, a newly generated defective pixel is mistaken as a foreign pixel. It is necessary to repeat the calibration several times and converge the averaged pixel value to a pixel value lower than the threshold value C. The designer may appropriately determine whether to give priority to accurately detecting a defective pixel due to crystallized a-Se or to give priority to expanding the detection range of the foreign pixel.

  (3) In the above-described embodiment, the FPD 3 includes a radiation (in the embodiment, X-ray) sensitive semiconductor and directly converts the incident radiation into a charge signal by the radiation sensitive semiconductor. However, instead of the radiation-sensitive type, it is equipped with a light-sensitive semiconductor and a scintillator, and the incident radiation is converted into light by the scintillator, and the converted light is converted into a charge signal by the light-sensitive semiconductor. It may be an indirect conversion type detector.

It is a block diagram of the X-ray diagnostic apparatus which concerns on an Example. It is the equivalent circuit of the flat panel type X-ray detector used for the X-ray diagnostic apparatus which concerns on an Example by the side view. 2 is an equivalent circuit of a flat panel X-ray detector in plan view according to the embodiment. It is a block diagram of the image processing part which concerns on an Example. It is a flowchart which shows a series of control sequences of the calibration which concerns on an Example. It is explanatory drawing which shows discrimination | determination of the foreign material pixel which concerns on an Example. It is a block diagram of the image processing part which concerns on the other Example of this invention. It is explanatory drawing which shows the threshold value calculation in the image process part which concerns on the other Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Top plate 2 ... X-ray tube 3 ... Flat panel type X-ray detector (FPD)
DESCRIPTION OF SYMBOLS 4 ... Top-panel control part 5 ... FPD control part 8 ... Image processing part 9 ... Controller part 31 ... 1st memory part 32 ... 2nd memory part 33 ... Memory part 34 ... 1st calculating part 35 ... 2nd calculating part 36 ... Foreign object detection unit 37... Pixel complementing unit 38... Amplification factor calculation unit 39.

Claims (5)

In a radiation imaging apparatus comprising a radiation irradiating means for irradiating radiation, and a radiation detecting means for detecting radiation irradiated by the radiation irradiating means,
A first calculation unit that calculates calibration irradiation data by taking an average of a plurality of image data detected by the radiation detection unit;
A second calculation unit that calculates calibration determination data by taking an average of the calibration irradiation data and the calibration irradiation data when the previous calibration was performed ;
A radiation imaging apparatus comprising: a foreign matter detection unit that detects a foreign matter on the radiation detection means by comparing a threshold value for judging foreign matter pixels and missing pixels of the radiation detection means and the calibration judgment data. . The radiation imaging apparatus according to claim 1,
The threshold value is a predetermined value based on an empirical rule. The radiation imaging apparatus according to claim 1,
A radiation imaging apparatus comprising: a threshold value calculation unit that calculates the threshold value from a distribution function of the calibration irradiation data each time calibration is performed. The radiation imaging apparatus according to any one of claims 1 to 3,
A radiation imaging apparatus comprising: a warning unit that warns of the presence of the foreign matter detected by the foreign matter detection unit. The radiation imaging apparatus according to any one of claims 1 to 4,
A radiation imaging apparatus comprising: a pixel complementing unit that complements a radiation detection value of a pixel that is prevented from being irradiated with radiation by the foreign matter.

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