JP4980769B2 - Radiation imaging apparatus and method - Google Patents

Description

  The present invention relates to a radiation imaging apparatus and a radiation imaging method for generating a medical image used for medical diagnosis by imaging a subject using radiation.

  Conventionally, imaging methods using radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet rays, etc.) have been used in various fields, and especially in the medical field, it is the most important for diagnosis. It has become one of the means. Since the first X-ray photography was realized, the X-ray photography method has been improved many times, and at present, a method combining a fluorescent screen and an X-ray film has become mainstream. On the other hand, in recent years, various digitized apparatuses such as CR (computed radiography), X-ray CT, ultrasound, and MRI have been put into practical use, and the construction of a diagnostic information processing system in a hospital has been promoted. I am trying to do.

  In recent years, in a radiation imaging apparatus, a flat panel detector (FPD) including a radiation detection device that captures an X-ray image by detecting X-rays transmitted through a subject at a plurality of detection points in a two-dimensional region is provided. It is used. Usually, the detection area of the flat panel detector is divided into a plurality of subareas, and a reading circuit is provided for each of the plurality of subareas. However, since the sensitivity of the radiation detection device and the amplification characteristic of the reading circuit are different from each other, a step is generated in the luminance of the image between two adjacent subareas.

  As a related technique, Patent Document 1 discloses an apparatus for forming an image composed of a plurality of sub areas. The apparatus evaluates image data from a detector including a plurality of sensor elements for generating image data, a reading unit associated with a sub-area of the image, and a contiguous image area of an adjacent sub-area and corrects the correction data. An analysis unit arranged to generate the image data and a correction unit arranged to correct illegal image data by the correction data.

According to the apparatus of Patent Document 1, a histogram is obtained for each subarea in order to analyze amplification characteristics for each reading unit, and a correction amount is calculated based on the histogram. The amplification mode can be corrected. Therefore, according to this apparatus, it is possible to correct a luminance level difference caused by a uniform variation in the sub-area. However, since there is a locality that the luminance step due to non-uniform fluctuation in the sub-area varies depending on the boundary position of the sub-area, the luminance step is not affected even if uniform correction is performed in the sub-area. It cannot be resolved.
JP 2001-197313 A (page 1-2, FIG. 2)

  Therefore, in view of the above points, the present invention provides a luminance step caused by non-uniform variation in a sub area in a radiation imaging apparatus using a flat panel detector having a detection area divided into a plurality of sub areas. The purpose is to correct.

In order to solve the above-described problem, a radiation imaging apparatus according to one aspect of the present invention is disposed in a radiation generation unit that generates radiation and a detection area divided into a plurality of sub-areas, and corresponds to the radiation dose. A detection element that accumulates charges and a plurality of reading circuits that are provided for each of the plurality of sub-areas and read information relating to charges accumulated in the detection elements in the respective sub-areas, and are generated by the radiation generator and the subject by detecting the transmitted through incident on the detection area radiation and outputs a detection signal and the radiation detector, and have contact to the detection signal output from the radiation detector a step occurring at the boundary of a plurality of sub-areas, a plurality of a step correction means for correcting, based on the detection signal obtained from a plurality of particular line in the sub area, calculate the level difference data based on the detection signal And performs filter processing for removing high frequency with respect to the stepped amount data, and a step correcting means for obtaining a correction amount based on the step amount data filtering process is performed.

In addition, a radiation imaging method according to one aspect of the present invention includes a detection element that is arranged in a detection area divided into a plurality of subareas and accumulates charges according to the radiation dose, and each subarea. By detecting radiation that has passed through the subject and entered the detection area using a radiation detector that has a plurality of reading circuits that read information relating to charges accumulated in the detection elements in the respective sub-areas. A radiation imaging method for performing imaging, wherein step (a) for calculating step amount data based on detection signals obtained from a plurality of specific lines in a plurality of sub-areas , and the step calculated in step (a) A step (b) for applying a filtering process to the quantity data to remove high frequency, and a filtering process is applied in step (b). Was based on the step amount data, and step (c) obtaining a correction amount in a plurality of sub-areas, based on the correction amount determined in step (c), a plurality of sub-in detection signal output from the radiation detector And (d) correcting a step generated at the boundary of the area.

  According to the present invention, the step generated at the boundary of the plurality of subareas in the detection signal output from the radiation detector is corrected based on the detection signal obtained from a specific line in the plurality of subareas. It is possible to correct a luminance step caused by non-uniform fluctuation.

Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings. The same constituent elements are denoted by the same reference numerals, and the description thereof is omitted.
FIG. 1 is a block diagram showing a configuration of a radiation imaging apparatus according to an embodiment of the present invention. In the present embodiment, the case where the present invention is applied to an X-ray mammography apparatus will be described. It is also possible to apply.

  The radiation imaging apparatus shown in FIG. 1 includes an X-ray imaging unit 10 and an X-ray imaging control unit (console) 20. The X-ray imaging unit 10 includes a tube voltage / tube current control unit 11, a high voltage generation unit 12, an X-ray tube 13, a filter 14, an X-ray detector 15, and an A / D converter 18. is doing. The tube voltage / tube current control unit 11 adjusts imaging conditions such as tube voltage and tube current according to the target value. The high voltage generator 12 generates a high voltage applied to the X-ray tube 13 under the control of the tube voltage / tube current controller 11.

  The X-ray tube 13 and the filter 14 constitute an X-ray generation unit that generates X-rays. In the X-ray tube 13, the X-ray transmission is determined by the tube voltage applied between the cathode and the anode, and the amount of X-rays generated is determined by the time integral value of the tube current flowing between the cathode and the anode. The The filter 14 is made of a material such as molybdenum (Mo) or rhodium (Rh), and selectively selects a desired wavelength component from among a plurality of wavelength components included in the X-rays generated by the X-ray tube 13. Make it transparent. X-rays generated by the X-ray tube 13 pass through the subject 1 through the filter 14 and are detected by the X-ray detector 15.

  The X-ray detector 15 is an FPD (Flat Panel Detector) that captures an X-ray image by detecting X-rays transmitted through a subject at a plurality of detection points in a two-dimensional region. On the detection surface of the FPD, there is provided an X-ray sensitive film such as selenium that absorbs X-rays and generates charges, and a detection element 16 that accumulates the charges generated by the X-ray sensitive films and outputs charge information. It has been. As the detection element 16, for example, a plurality of TFTs (thin film transistors) that are arranged in a two-dimensional matrix and are scanned by an electric signal to release accumulated charges, or accumulated when scanned by reading light. An optical reading type detection element that discharges electric charge is used.

  When X-rays enter the FPD, the X-rays are converted into electric charges in the X-ray sensitive film, and the electric charges are accumulated in the detection element 16. The reading unit 17 provided in the FPD reads out the charge information output from the detection element 16 based on the charge accumulated in the detection element 16, and generates a detection signal having a level corresponding to the X-ray irradiation dose. . The A / D converter 18 converts the analog detection signal output from the reading unit 17 into a digital signal (detection data).

  FIG. 2 is a side view showing an appearance of the X-ray imaging unit shown in FIG. The X-ray imaging unit 10 includes an arm unit 2 in which an X-ray tube 13 and an X-ray detector 15 are installed, a base 3, and a shaft unit 4 that holds the arm unit 2 in a rotatable state on the base 3. Including. The X-ray detector 15 detects X-rays irradiated from the X-ray tube 13 and transmitted through the subject 1 (breast), and outputs a detection signal having a level corresponding to the X-ray irradiation dose. A detection signal output from the X-ray detector 15 is input via a cable 19 to an A / D converter 18 (FIG. 1) installed inside the arm unit 2.

  The arm unit 2 is provided with a position adjusting unit 5 that moves the X-ray detector 15 in the horizontal direction in the drawing, and a direction adjusting unit 6 that rotates the X-ray detector 15 in the horizontal plane. Further, a compression plate 8 is attached to the arm portion 2 via a compression plate drive unit 7. The compression plate 8 is installed in parallel to the X-ray detector 15 so as to be movable in the vertical direction in the drawing. The compression plate 8 and the X-ray detector 15 sandwich the subject 1 (breast), and X-ray imaging is performed with the breast thickness uniform.

  FIG. 3 is a diagram showing details of the X-ray detector shown in FIGS. 1 and 2. In FIG. 3, the X-axis direction is the main scanning direction, and the Y-axis direction is the sub-scanning direction. The detection area of the X-ray detector 15 is divided into a plurality of sub-areas (in FIG. 3, five sub-areas 151 to 155 are shown) in the main scanning direction. A plurality of reading circuits 17 (five reading circuits 171 to 175 are shown in FIG. 3) are connected to each other. As each of the reading circuits 171 to 175, for example, a semiconductor integrated circuit (ASIC) designed and manufactured for a specific application is used. Using the reading circuits 171 to 175, reading scanning is performed in the sub-scanning direction in the respective sub-areas 151 to 155. In the present application, the “sub-area” means any one of an image region having a different detection area in the radiation detection device, a semiconductor integrated circuit, an A / D converter substrate, a substrate on which the circuit is mounted, or These image areas have different combinations.

  Here, there is a difference in the reference level of the detection signal within or between sub-areas due to non-uniformity of the X-ray sensitive film, sensitivity variations in the detection elements, and variations in amplification characteristics in a plurality of reading circuits. An offset occurs. As a result, in the radiographic image, a luminance step is generated as an artifact.

  For example, an offset occurs between the pixel (detection point) P1 on the line L1 near the subarea boundary in the subarea 152 and the pixel P2 on the line L2 near the subarea boundary in the subarea 153, and the line If an offset occurs between other pixels on L1 and L2, a luminance step occurs at the boundary between the subarea 152 and the subarea 153. However, due to the variation of each element in the X-ray detector 15, an offset is generated between a plurality of pixels on the line L1, and an offset is generated between a plurality of pixels on the line L2. It is inappropriate to uniformly correct the level of the detection signal obtained from the above.

  Referring back to FIG. 1, the X-ray imaging control unit (console) 20 corrects a step between subareas in detection data input from the X-ray imaging unit 10, and generates a step data correction unit 21. An image processing unit 26 that performs image processing on the image data output from the step correction unit 21, a D / A converter 27, a display unit 28, a console 29, a control unit 30, and a storage unit 31 are provided. is doing.

  As a functional block, the step correction unit 21 sequentially compares detection data from pixels on two lines in two adjacent subareas, calculates a step amount between those lines, and generates step amount data. A step amount calculation unit 22, a filter unit 23 that performs a filter process on the step amount data, a correction amount calculation unit 24 that generates correction amount data based on the step amount data subjected to the filter process, and correction amount data And a correction unit 25 that corrects the level difference between the sub-areas and generates image data.

  The image processing unit 26 generates image data for display by performing necessary image processing such as gradation processing on the image data output from the step correction unit 21. Further, the image processing unit 26 may perform processing for suppressing unevenness in the direction parallel to the boundaries of the plurality of sub-areas on the image data that has been subjected to the step correction by the step correction unit 21.

  The console 29 is used by an operator to operate the radiation imaging apparatus. The control unit 30 is configured by a central processing unit (CPU) and software for causing the CPU to perform various processes, and controls each unit based on the operation of the operator. In the present embodiment, the level difference correction unit 21 and the image processing unit 26 are also configured by a CPU and software, but may be configured by a digital circuit or an analog circuit. The above software is stored in the storage unit 31 constituted by a hard disk or a memory.

  Next, data processing in the radiation imaging apparatus shown in FIG. 1 will be described with reference to FIGS. 3 and 4. FIG. 4 is a flowchart showing the flow of data processing in the radiation imaging apparatus shown in FIG.

  In step S1 of FIG. 4, the detection data output from the X-ray imaging unit 10 is divided for each sub-area. In the following, it is assumed that 128 pixels (lines) are included in the X-axis direction in each sub-area.

  In step S2, the step amount calculation unit 22 sequentially compares detection data from pixels on two lines in two adjacent subareas defined by the step amount calculation position parameter, and the step amount between these lines. (Offset) is calculated to generate step amount data. Thereby, the step amount between the sub area 151 and the sub area 152 shown in FIG. 3, the step amount between the sub area 152 and the sub area 153, the step amount between the sub area 153 and the sub area 154, and The step amount between the sub area 154 and the sub area 155 is calculated.

  The step amount calculation position parameter defines the position (left pixel position, right pixel position) in the X-axis direction of the pixel on the line used for the step amount calculation. Usually, the pixels on the two lines closest to the sub-area boundary in two adjacent sub-areas are used for the step amount calculation, so the value indicating the position of these pixels in the X-axis direction is the step amount calculation position. It is set as the default value of the parameter. The reason for performing the step correction based on the two lines closest to the sub-area boundary is that when there is an image signal whose luminance changes in a direction orthogonal to the sub-area boundary, the line used for the step amount calculation is the sub-area boundary. This is because the amount of change in the image signal increases and the error in the step amount calculation increases.

  However, since it is inappropriate to use pixels on an abnormal defect line for level difference correction, the level difference calculation unit 22 is based on line inspection information indicating whether each line is a normal line or a defective line. If any of the lines used for level difference correction is a defective line, the value of the level difference calculation position parameter is changed. For example, the line inspection information is acquired by the control unit 30 periodically inspecting detection signals from the respective lines, and stored in the storage unit 31.

  In step S <b> 3, the filter unit 23 performs a filtering process on the step amount data representing the step amount between the two lines calculated by the step amount calculation unit 22. This filtering process is performed to remove high-frequency components included in the step amount data (step amount profile). In this embodiment, the median filtering process is performed based on the filter pixel number parameter. . If the correction amount is calculated based on the step amount data including the high-frequency component due to noise or the image signal, the correction amount also includes the high-frequency component, and the high-frequency component may cause horizontal stripes in the image. is there.

  The high-frequency component to be removed is variable by setting the filter pixel number parameter. The filter pixel number parameter defines an appropriate window size (number of pixels) in the median filter processing so as to prevent horizontal streak-like artifacts and not increase the processing time so much, and the default value is 201 pixels. ing. Note that the end point processing in the median filter processing is performed by changing the window size in accordance with the number of pixels at the end of the image.

  For example, in the case of a median filtering process with a window size of 201 pixels, the window size is 1 pixel (self) for the pixel at the end of the image, and the window size is 3 pixels (self and The window size is 5 pixels for the adjacent pixels. As described above, the window size in the median filter processing changes as 1, 3, 5,..., 201, 201, 201,.

  In step S4, the correction amount calculation unit 24 multiplies the value of the step difference data subjected to the filter processing by a factor of 1/2 to obtain a correction amount for one of the two adjacent subareas. In step S5, a correction amount for the other of the two adjacent subareas is obtained by multiplying the value of the step amount data multiplied by 1/2 by -1. That is, the level difference correction is performed in half for these two sub areas. Thereby, it is possible to correct the luminance step while reducing the influence on the original image information. In this way, for example, correction amounts for the sub-areas 152 and 153 shown in FIG. 3 are obtained.

  Further, in step S6, the correction amount calculation unit 24 determines the left of all lines in the left subarea (for example, the subarea 152) based on the detection data for each subarea and the correction intensity table in the left direction. Direction correction amount data is obtained, and in step S7, the right direction for all lines in the right sub-area (for example, sub-area 153) based on the detection data for each sub-area and the right-side correction intensity table. The correction amount data is obtained.

The correction intensity table defines correction intensity distributions for pixels on all lines in one sub-area. For example, when the number of data bits in the correction intensity table is 16 bits (2 ¹⁶ = 65536), the correction intensity table in the left direction corresponds to the pixel number (line number) n in the X-axis direction. The coefficient K (n) = INT {65535n / (128-1) +0.5} is stored. Here, pixel numbers n = 0, 1, 2,..., 127, and “INT” is a function for obtaining an integer part.

  According to this correction intensity table, the correction coefficient K (0) = 0 for the leftmost pixel with pixel number n = 0, and the correction coefficient K (127) = 65535 for the rightmost pixel with pixel number n = 127. . As a result, in the left sub-area, the correction amount at the leftmost pixel becomes zero, and the correction amount of the step can be increased as it approaches the sub-area boundary. Can be prevented from propagating to other sub-areas.

  However, in the leftmost subarea (subarea 151), there is no further left subarea, so the correction coefficient K (n) may be constant. Similarly, in the rightmost subarea (subarea 155), there is no further right subarea, so the correction coefficient K (n) may be constant.

  In step S8, the correction unit 25 adds the left-direction correction amount data obtained in step S6 to the detection data for the left sub-area, and the right direction obtained in step S7 for the right sub-area. The correction amount data is added to the detection data. Thereby, image data in which a luminance step in the image is corrected is generated.

  If the step amount is greater than or equal to a predetermined value, it is considered that the step amount is based on the image information of the subject. Therefore, the correction unit 25 determines the step amount based on the step amount threshold parameter. It is determined whether or not it is equal to or greater than the threshold value. As a result, when the step amount is greater than or equal to a predetermined threshold value, the step is not corrected. In the present embodiment, the default value of the step amount threshold parameter is 4000.

  Thereafter, the image processing unit 26 may perform processing for suppressing unevenness in a direction parallel to the boundaries of the plurality of sub-areas. This process is performed, for example, by performing an FFT (Fast Fourier Transform) process on the image data and reducing only a specific frequency component. As a result, unevenness that cannot be removed by the step correction can be reduced.

  The present invention can be used in a radiation imaging apparatus that generates a medical image by imaging a subject using radiation.

It is a block diagram which shows the structure of the radiation imaging device which concerns on one Embodiment of this invention. It is a side view which shows the external appearance of the X-ray imaging part shown in FIG. It is a figure which shows the detail of the X-ray detector shown in FIG.1 and FIG.2. It is a flowchart which shows the flow of the data processing in the radiation imaging device shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Subject 2 Arm part 3 Base 4 Axis part 5 Position adjustment part 6 Direction adjustment part 7 Compression board drive part 8 Compression board 10 X-ray imaging part 11 Tube voltage / tube current control part 12 High voltage generation part 13 X-ray tube DESCRIPTION OF SYMBOLS 14 Filter 15 X-ray detector 151-155 Subarea 16 Detection element 17 Reading part 171-175 Reading circuit 18 A / D converter 20 X-ray imaging control part (console)
DESCRIPTION OF SYMBOLS 21 Level | step difference correction | amendment part 22 Level difference amount calculation part 23 Filter part 24 Correction amount calculation part 25 Correction | amendment part 26 Image processing part 27 D / A converter 28 Display part 29 Console 30 Control part 31 Storage part

Claims (11)

A radiation generator for generating radiation;
A detection element that is arranged in a detection area divided into a plurality of sub-areas and accumulates electric charges according to the radiation dose, and is provided for each of the plurality of sub-areas, and is accumulated in the detection elements in each sub-area. A radiation detector that reads a plurality of reading circuits that read information relating to the charges, detects radiation that has been generated by the radiation generator and transmitted through the subject and incident on the detection area, and outputs a detection signal;
Step correction means for correcting, based a step occurring at the boundary of the plurality of sub-area have you on the detection signal outputted from the radiation detector, the detection signals obtained from a plurality of particular line in the plurality of sub-areas Then, the step amount data is calculated based on the detection signal, the filter processing for removing the high frequency is performed on the step amount data, and the correction amount is obtained based on the step amount data subjected to the filter processing. The step correction means ;
A radiation imaging apparatus comprising: Control means for acquiring line inspection information indicating whether each line of the radiation detector is a normal line or a defective line by periodically inspecting a detection signal obtained from each line of the radiation detector. In addition,
The step difference correction unit calculates step amount data based on detection signals obtained from two lines closest to each other excluding a defective line represented by line inspection information in two adjacent subareas. The radiation imaging apparatus according to 1. Filter characteristic in the filtering process is variable by the setting of the parameters, a radiation imaging apparatus according to claim 1 or 2, wherein. The step correction means obtains a correction amount for one of two adjacent sub-areas by multiplying the step amount represented by the step amount data by 1/2 , and the step amount represented by the step amount data the multiplying by -1/2, obtain a correction amount for the other of the two adjacent sub-areas, the radiation imaging apparatus of any one of claims 1-3. The step correction means, depending on the distance from the boundary of the sub-area to change the correction amount, the radiation imaging apparatus of any one of claims 1-4. The step correction means suppresses unevenness in a direction parallel to the boundaries of the plurality of sub-areas after correcting the steps generated at the boundaries of the plurality of sub-areas in the detection signal output from the radiation detector; The radiation imaging apparatus of any one of Claims 1-5 . A detection element that is arranged in a detection area divided into a plurality of sub-areas and accumulates electric charges according to the radiation dose, and is provided for each of the plurality of sub-areas, and is accumulated in the detection elements in each sub-area. A radiation imaging method for performing imaging by detecting radiation that has passed through a subject and is incident on the detection area, using a radiation detector that has a plurality of reading circuits that read information about the charges,
Based on the detection signal obtained from a plurality of particular line in the plurality of sub-areas, and the step (a) for calculating the level difference data,
(B) performing a filtering process for removing high frequency on the step amount data calculated in step (a);
Step (b) based on the step amount data filtering process is performed in a step (c) obtaining a correction amount in the plurality of sub-areas,
A step (d) of correcting a step generated at a boundary of the plurality of sub-areas in a detection signal output from the radiation detector based on the correction amount obtained in the step (c);
A radiation imaging method comprising: Prior to step (a), line inspection information indicating whether each line of the radiation detector is a normal line or a defective line is obtained by inspecting a detection signal obtained from each line of the radiation detector. And further comprising the step of obtaining
Step (a) includes calculating step amount data based on detection signals obtained from two lines closest to each other except for a defective line represented by line inspection information in two adjacent subareas. The radiation imaging method according to claim 7 . Step (c), by half a step amount represented by the step amount data, obtain a correction amount for one of the two adjacent sub-areas, step amount represented by the level difference data the multiplying by -1/2 includes determining a correction amount for the other of the two adjacent sub-areas, according to claim 7 or 8 radiographic imaging method according. Step (c) comprises changing the correction amount in accordance with the distance from the boundary of sub-areas, radiation imaging method according to any one of claims 7-9. After step (d), a radiation imaging method according to any one of claims 7 to 10, further comprising the step (e) inhibits the parallel direction of the unevenness in the boundary of the plurality of sub-areas.

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