X-ray phase imaging apparatus and image processing method

Abstract

To provide an X-ray phase imaging apparatus and an image processing method for improving contrast and visibility of an image photographed by a phase contrast method.SOLUTION: According to an aspect of the present invention, there is provided an X-ray phase imaging apparatus comprising an X-ray source, a detector configured to detect X-rays emitted from the X-ray source to a subject, and an image processing unit configured to generate an image of the subject, the image processing unit includes a storage unit in which a plurality of noise correction images for correcting a luminance variation between pixels are stored separately for brightness and darkness, and an image generation unit that generates a luminance image of the subject by irradiating the subject with X-rays, generates a plurality of corrected images of the subject by correcting the luminance image according to the noise correction image, and generates one subject image by selecting the plurality of corrected images according to a threshold value.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an X-ray phase imaging apparatus and an image processing method. [Background technology]

[0002] The invention described in Patent Document 1 discloses an invention relating to an X-ray imaging device that acquires phase contrast information and absorption contrast information of a subject separately from X-rays detected by a detector, weights the phase contrast information, and adds the weighted information and the absorption contrast information to generate composite image information. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-208189 Summary of the Invention [Problem to be solved by the invention]

[0004] The contrast of the images taken by the phase contrast method was improved.

[0005] Therefore, the present invention has been made to solve the above-mentioned problems, and aims to provide an X-ray phase imaging device and an image processing method that improve the contrast and visibility of images taken using the phase contrast method. [Means for solving the problem]

[0006] An X-ray phase imaging device according to one embodiment of the present invention comprises an X-ray source, a detector that detects X-rays irradiated from the X-ray source to an object, and an image processing unit that generates an image of the object, wherein the image processing unit has a memory unit in which multiple noise correction images for correcting brightness variations between pixels are stored, separated by brightness, and an image generation unit that includes: irradiating the object with X-rays to generate a brightness image of the object; generating multiple corrected images of the object by correcting the brightness image according to the noise correction image; and selecting the multiple corrected images using a threshold to generate a single object image.

[0007] An image processing method according to one aspect of the present invention is an image processing method that places an object between an X-ray source and a detector, detects X-rays irradiated onto the object from the X-ray source, and generates an image of the object in an image processing unit, and includes the steps of generating multiple noise correction images for light and dark to correct brightness variations between pixels, irradiating the object with X-rays to generate a brightness image of the object, correcting the brightness image according to the noise correction image to generate multiple corrected images of the object, and selecting the multiple corrected images using a threshold value to generate a single object image. [Effects of the Invention]

[0008] According to the present invention, it is possible to improve the contrast of an image captured by the phase contrast method and thereby improve visibility. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic diagram of an X-ray phase imaging apparatus according to an embodiment of the present invention. [Figure 2] This is an image of photography using the absorption contrast method. [Figure 3] This is an image of photography using the absorption contrast method. [Figure 4] FIG. 1 is an image diagram of imaging using the phase contrast method. [Figure 5]1 is a block diagram of an X-ray phase imaging apparatus according to an embodiment of the present invention. [Figure 6] 1A to 1C are conceptual diagrams for explaining steps of an image processing method according to the present embodiment. [Figure 7] 1A to 1C are conceptual diagrams for explaining steps of an image processing method according to the present embodiment. [Figure 8] 1A to 1C are conceptual diagrams for explaining steps of an image processing method according to the present embodiment. [Figure 9] 1A to 1C are conceptual diagrams for explaining steps of an image processing method according to the present embodiment. [Figure 10] 1A to 1C are conceptual diagrams for explaining steps of an image processing method according to the present embodiment. [Figure 11] 1A to 1C are conceptual diagrams for explaining steps of an image processing method according to the present embodiment. [Figure 12] 1A to 1C are conceptual diagrams for explaining steps of an image processing method according to the present embodiment. [Figure 13] FIG. 2 is a conceptual diagram of a subject image (phase contrast image) generated by the image processing method according to the present embodiment. [Figure 14] 14 is a graph showing the luminance on line AA' of the image shown in FIG. 13. [Figure 15] FIG. 2 is a flow chart of image processing method steps according to the present embodiment; [Figure 16] FIG. 10 is a flowchart of steps of an image processing method according to a comparative example. [Figure 17] FIG. 10 is an image diagram of a subject image (phase contrast image) generated by an image processing method according to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an X-ray phase-shift imaging apparatus and an image processing method according to embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiments, and can be modified appropriately within the scope of the present invention.

[0011] <Overall configuration diagram of the X-ray phase imaging apparatus 1> FIG. 1 is a schematic diagram of an X-ray phase imaging apparatus 1 according to the present embodiment. As shown in FIG. 1, the X-ray phase imaging apparatus 1 includes an X-ray source 2, a detector 4 that detects X-rays irradiated from the X-ray source 2 to a subject 3, and an image processing unit 5 that generates an image.

[0012] The X-ray source 2 is not particularly limited as long as it is a device that generates X-rays, and an existing X-ray source (X-ray tube) can be applied. As shown in FIG. 1, X-rays are irradiated in the direction from the X-ray source 2 toward the subject 3.

[0013] The detector 4 is not particularly limited as long as it is a detector that can detect X-rays transmitted through the subject 3, and an existing device can be applied. For example, the detector 4 is a flat panel detector, and a plurality of conversion elements and a plurality of pixel electrodes are arranged in an array in the X direction and the Y direction at a pixel pitch. The detector 4 converts the detected X-rays into an electrical signal, reads the converted electronic signal as an image signal, and outputs the image signal to the image processing unit 5. The image processing unit 5 generates a phase contrast image based on the image signal output from the detector 4.

[0014] The X-ray phase imaging apparatus 1 of the present embodiment is characterized in that it performs image processing that improves the contrast of an image taken by the phase contrast method without degrading the image quality as compared with the conventional method.

[0015] First, the background of the X-ray phase imaging apparatus and the image processing method of the present embodiment will be described.

[0016] <Background of the X-ray phase imaging apparatus and the image processing method of the present embodiment> Conventionally, a method of nondestructively inspecting the inside of an opaque object by using the property that X-rays pass through an object has been put into practical use.

[0017] Figure 2 shows an image captured using the absorption contrast method. As shown in Figure 2, the subject 6 has a structure in which, for example, metal 8 is embedded inside an opaque resin material 7. In the absorption contrast method, when X-rays are passed through the subject 6, the amount of X-ray absorption (or transmission) is used to generate contrast in the captured image. This contrast makes it possible to determine whether or not cracks or the like have occurred in the metal 8.

[0018] The absorption contrast method is effective for objects with large atomic numbers that easily absorb X-rays, making it possible to inspect defects in metals 8, as shown in Figure 2.

[0019] On the other hand, the absorption rate is low for subjects made of light elements such as hydrogen and carbon. For this reason, as shown in Figure 3, for example, for a subject 3 made of opaque resin material 7 that has voids 3a inside, the difference in X-ray absorption rate is small, making it difficult to obtain contrast and distinguish the voids 3a inside the subject 3.

[0020] The phase contrast method is a method for photographing objects for which it is difficult to obtain contrast. As shown in Figure 4, the phase contrast method uses the phase shift (refraction of X-rays) that occurs when X-rays pass through an object 3 to perform photography. As shown in Figure 4, the phase shift differs between areas where X-rays pass through voids 3a and areas where there are no voids 3a, which improves visibility somewhat compared to the absorption contrast method, but the low contrast remains an issue.

[0021] As a result of extensive research, the inventor has succeeded in improving the contrast of an image by preparing multiple noise correction images for light and dark, generating multiple corrected images of the subject according to the noise correction images, and selecting the corrected images using an arbitrary threshold value to generate a single image of the subject.

[0022] <Configuration of the image processing unit and image processing method according to this embodiment> 5 is a block diagram of the X-ray phase imaging apparatus 1 according to the present embodiment. As shown in FIG. 5, the image processing unit 5 includes an image generating unit 10, a threshold setting unit 11, and a storage unit 12.

[0023] The image processing unit 5 is not limited to this, but may be configured as, for example, a part of a processor of a personal computer connected to the detector 4. Therefore, although the image processing unit 5 is illustrated in FIG. 5 as being divided into multiple functional units, these functional units do not need to be realized by clearly separated programs. The image generating unit 10 shown in FIG. 5 generates an image based on the image signal detected by the detector 4.

[0024] The threshold setting unit 11 sets a threshold as a reference value for selecting a plurality of corrected images so that bright areas become brighter and dark areas become darker. The threshold can be set arbitrarily and is not limited to any particular value. Specific examples will be described later. The storage unit 12 stores the two-dimensional numerical map, the image data generated by the image generation unit 10, and the like.

[0025] Next, an image processing method according to this embodiment will be described. Figures 6 to 13 are conceptual diagrams showing steps of the image processing method. Figure 14 is a graph showing the luminance on line AA' of the image shown in Figure 13. Figure 15 is a flowchart of steps of the image processing method according to this embodiment.

[0026] In Fig. 6A, multiple dark images d(p, x, y) are acquired without irradiating X-rays from the X-ray source 2. Although the number of images taken is not limited, in this embodiment, the number of images taken is set to 30. Multiple images can be taken continuously at short time intervals.

[0027] Next, the number of each dark image d is p (a variable from 1 to P), the horizontal position in the image is x, and the vertical position is y, and the average luminance image DarkVal(x, y) shown in Figure 6B is generated using the following (Equation 1) (step ST1 in Figure 15).

[0028]

number

[0029] Next, in Fig. 7A, multiple bright images b(p, x, y) are acquired without placing a subject and irradiating X-rays from the X-ray source 2. The number of bright images is not limited, but in this embodiment, it is set to 30, the same as the dark images in Fig. 6A.

[0030] Next, the number of each bright image b is set to p (p=variable from 1 to P), the horizontal position in the image is set to x, and the vertical position is set to y, and an average brightness image BrightVal(x, y) shown in FIG. 7B is generated using the following (Equation 2) (step ST2 in FIG. 15).

[0031]

number

[0032] Next, using the bright image b(p, x, y) acquired in FIG. 7A and the average brightness image BrightVal(x, y) acquired in FIG. 7B, the standard deviation StdVal(x, y) of brightness at each pixel is calculated using the following (Equation 3) (step ST3 in FIG. 15).

[0033]

number

[0034] Since it is a standard deviation, it is not an image but a two-dimensional numerical map. The standard deviation StdVal(x, y) is stored in the storage unit 12 shown in FIG.

[0035] Next, in Fig. 8, DarkVal(x, y) obtained in step ST1 is subtracted from BrightVal(x, y) obtained in step ST2 as shown in the following (Equation 4). The result is set as BrightVal2(x, y) (step ST4 in Fig. 15).

[0036]

number

[0037] By calculating BrightVal2(x,y), normal noise such as noise caused by the heat of the X-ray camera and variations between pixels can be suppressed.

[0038] Next, as shown in FIG. 9, the average brightness AverageVal of the entire image is calculated from BrightVal2(x, y) using the following (Equation 5) (step ST5 in FIG. 15).

[0039]

number

[0040] Next, the noise correction value GainVal(x, y) is calculated by the following (Equation 6) using the average brightness AverageVal obtained in step ST5 and BrightVal2(x, y) obtained in step ST4.

[0041]

number

[0042] GainVal(x,y) indicates how bright or dark an image is compared to the average brightness, and by using GainVal(x,y), it is possible to make corrections to reduce variations in brightness between pixels. In practice, the noise correction value is multiplied by a value corresponding to the number of bits in the image format in order to save it as an image. For example, if the image is 14 bits, it is multiplied by 2 14 , if it is 16 bits, then 2 16 Apply. The GainVal(x, y) obtained in step ST6 is referred to as a "normal noise correction image."

[0043] Next, in step ST7, noise correction values ​​GainVal_H(x,y) and GainVal_L(x,y) are calculated using the average brightness AverageVal calculated in step ST5, BrightVal2(x,y) calculated in step ST4, and the standard deviation StdVal(x,y) calculated in step ST3, according to the following equations (7) and (8).

[0044]

number

[0045]

number

[0046] GainVal_H(x,y) and GainVal_L(x,y) are also saved as images.

[0047] As shown in (Equation 7), when calculating GainVal_H(x,y), the standard deviation StdVal(x,y) is added to the average brightness AverageVal, so that GainVal_H(x,y) can be corrected to be brighter than GainVal(x,y).

[0048] Furthermore, as shown in (Equation 8), when calculating GainVal_L(x,y), the standard deviation StdVal(x,y) is subtracted from the average brightness AverageVal, so that GainVal_L(x,y) can be corrected to be darker than GainVal(x,y).

[0049] GainVal_H(x,y) obtained in step ST7 is referred to as a "noise correction image brighter than a normal noise correction image," and GainVal_L(x,y) is referred to as a "noise correction image darker than a normal noise correction image." Each noise correction image is stored in the storage unit 12.

[0050] Steps ST1 to ST7 are performed prior to capturing an image of the subject 3 to be observed. The noise correction images stored in the storage unit 12 can be updated periodically or when the X-ray phase imaging device 1 is started.

[0051] Next, as shown in FIG. 1, an object 3 is placed between the X-ray source 2 and the detector 4, and P (plural) phase contrast images p(p, x, y) are captured as shown in FIG. 10. While the object 3 is not limited, the image processing method of this embodiment is suitable for improving the contrast of a phase contrast image of an object 3 having an internal void 3a, as shown in FIGS. 3 and 4. While the number of images captured is not limited, it is preferable to capture several tens of images to obtain an appropriate contrast improvement effect. At this time, the irradiation settings of the X-ray source 2 and the settings of the detector 4 are not changed. Furthermore, the object 3 is also fixed. In this embodiment, the number of images captured is 30. Multiple images can be captured continuously at short time intervals.

[0052] Then, DarkVal(x,y) calculated in step ST1 is subtracted from each phase contrast image p(p,x,y). This is defined as PixelVal(p,x,y) (step ST8 in Figure 15). In this way, by acquiring multiple phase contrast images p(p,x,y), it is possible to suppress normal noise such as noise due to the heat of the X-ray camera and variations between pixels.

[0053] Next, in FIG. 11, one average luminance image PixelVal(x,y) is generated from PixelVal(p,x,y) obtained in FIG. 10 by the following (Equation 9) (step ST8 in FIG. 15).

[0054]

number

[0055] Next, using the average luminance image PixelVal(x, y) calculated in step ST8 and GainVal(x, y) calculated in step ST6, a corrected image CorrVal(x, y) of the subject 3 is generated by the following (Equation 10) (step ST9 in Figure 15).

[0056]

number

[0057] Here, Offset can be determined arbitrarily. Note that a higher Offset results in a brighter corrected image, and a lower Offset results in a darker corrected image. This Offset is the same value as the Offset expressed by the following (Equation 11) and (Equation 12). Note that, although the luminance distribution of each corrected image is illustrated in FIG. 14, the Offset was set to 0 when obtaining these corrected images. Here, the corrected image CorrVal(x, y) will be referred to as a “normal corrected image.” Fig. 12A is an image diagram of a normal corrected image.

[0058] Next, using the average luminance image PixelVal(x,y) calculated in step ST8 and GainVal_H(x,y) calculated in step ST7, a corrected image CorrVal_H(x,y) of the subject 3 is generated by the following (Equation 11) (step ST10 in Figure 15).

[0059]

number

[0060] In addition, using the average luminance image PixelVal(x, y) calculated in step ST8 and GainVal_L(x, y) calculated in step ST7, a corrected image CorrVal_L(x, y) of the subject 3 is generated by the following (Equation 12) (step ST10 in Figure 15).

[0061]

number

[0062] Here, the corrected image CorrVal_H(x,y) is referred to as the "corrected image corrected to be brighter," and the corrected image CorrVal_L(x,y) is referred to as the "corrected image corrected to be darker." A corrected image corrected to be brighter is overall brighter than a normal corrected image, and a corrected image corrected to be darker is overall darker than a normal corrected image. FIG. 12B is an image diagram of a corrected image corrected to be brighter, and FIG. 12C is an image diagram of a corrected image corrected to be darker. In this way, three corrected images with different brightness levels are generated from one average luminance image PixelVal(x, y).

[0063] Next, as a final image, one object image OutputVal (second image) shown in Fig. 13 is obtained. The luminance distribution of the line AA' shown in Fig. 13 is shown in Fig. 14.

[0064] In addition to the luminance distribution of the subject image OutputVal, FIG. 14 also illustrates CorrVal, CorrVal_H, and CorrVal_L as the corrected images obtained in steps ST9 and ST10.

[0065] In this embodiment, CorrVal, CorrVal_H, and CorrVal_L are selected using thresholds A and B shown in FIG. 14 to generate one object image OutputVal. In this embodiment, the threshold setting unit 11 sets two thresholds A and B. The thresholds A and B can be determined arbitrarily.

[0066] As shown in steps ST11 and ST12 in FIG. 15, for pixel regions where CorrVal(x, y)>threshold A, CorrVal_h(x, y) is selected as OutputVal(x, y).

[0067] For the pixel region where CorrVal(x, y) < threshold A, transition from step ST11 to step ST13 and determine whether CorrVal(x, y) < threshold B. If CorrVal(x, y) < threshold B, transition to step ST14 and select CorrVal_L(x, y) as OutputVal(x, y).

[0068] In step ST13, for the pixel region where CorrVal(x, y) > threshold B, transition to step ST15. That is, select CorrVal(x, y) where threshold B < CorrVal(x, y) < threshold A as OutputVal(x, y).

[0069] In this way, by selecting the corrected images of each pixel used as OutputVal(x, y) in steps ST11 to ST15 and combining the selected corrected images into one, a single subject image OutputtVal shown in FIGS. 13 and 14 can be obtained. On the other hand, the flowchart of FIG. 16 shows a comparative example, and the subject image FFCVal generated based on the flowchart of FIG. 16 is shown in FIG. 17.

[0070] Steps 20 to ST24 shown in FIG. 16 can be obtained in steps ST1, 2, 4, 5, 6, 8 described above.

[0071] In step ST25 of FIG. 16, a subject image FFCVal is generated by the following (Equation 13).

[0072]

Equation

[0073] Comparing the image of the embodiment shown in FIG. 13 with the image of the comparative example shown in FIG. 17, it can be seen that the contrast of the embodiment is higher than that of the comparative example.

[0074] Comparing the image processing of this embodiment with the image processing of the comparative example, in this embodiment (1) Providing multiple noise correction images for brightness correction, (2) generating a plurality of corrected images of the subject corrected according to the noise correction image, and finally obtaining one image of the subject; (3) When generating one subject image, a luminance of the corrected image is selected based on an arbitrary threshold value. These points are different from those of the comparative example.

[0075] Furthermore, in this embodiment, when generating the noise correction image (1), the standard deviation of the luminance at each pixel is calculated based on the luminance image obtained by irradiating the image with X-rays without placing a subject, and this standard deviation is used to obtain multiple noise correction images for light and dark. In this way, using the standard deviation of the luminance to generate noise correction images that are brighter and / or darker than normal noise correction images is one of the features of the image processing method of this embodiment.

[0076] By carrying out the image processing steps (1) to (3) above, it is possible to generate a single image in which bright areas are corrected to be brighter and dark areas are corrected to be darker, thereby improving the contrast compared to the comparative example.

[0077] Furthermore, in this embodiment, all acquired luminance images are averaged and used, which effectively suppresses normal noise such as noise due to the heat of the X-ray camera and variations between pixels.

[0078] Although the disclosed embodiments and their advantages have been described in detail above, it will be apparent to those skilled in the art that various modifications, additions, and omissions may be made thereto without departing from the scope of the present invention as clearly set forth in the claims.

[0079] For example, in the above embodiment, the noise correction images and the correction images of the subject are of three types: a normal image, an image brighter than normal, and an image darker than normal, but they may be of two types or four or more types.

[0080] In the above embodiment, two thresholds are used, but increasing the number can increase the contrast. In particular, if the subject 3 has a complex internal shape, increasing the thresholds can enable the internal shape to be captured more clearly. [Industrial Applicability]

[0081] The X-ray phase-contrast imaging device and image processing method of the present invention can improve the contrast of images captured by the phase contrast method, thereby improving the discernibility of voids and other objects present inside the subject. The X-ray phase-contrast imaging device of the present invention can be widely used in fields ranging from industrial to medical. [Explanation of symbols]

[0082] 1: X-ray phase imaging device 2:X-ray source 3, 6: Subject 3a: void 4: Detector 5: Image processing section 7: Resin material 8: Metal 10: Image generation unit 11: Threshold setting section 12: Storage section

Claims

1. An X-ray phase imaging apparatus, comprising: an X-ray source; a detector that detects X-rays irradiated from the X-ray source onto the subject; an image processing unit that generates an image of the subject, The image processing unit includes a storage unit in which a plurality of noise correction images for correcting luminance variations between pixels are stored, separated by brightness; irradiating the subject with X-rays to generate a luminance image of the subject; generating a plurality of corrected images of the subject by correcting the luminance image in accordance with the noise correction image; and and an image generation unit that selects the plurality of corrected images using a threshold value to generate one subject image. An X-ray phase imaging apparatus characterized by:

2. the storage unit includes, as the noise correction image, a normal noise correction image generated from a luminance image obtained by irradiating the X-rays without placing the subject, and a noise correction image brighter than the normal noise correction image and / or a noise correction image darker than the normal correction image, generated using the luminance image and a standard deviation of luminance at each pixel based on the luminance image; The image generation unit generates, as the corrected image, a normal corrected image corrected in accordance with the normal noise correction image, a corrected image corrected to be bright in accordance with the bright noise correction image, and / or a corrected image corrected to be dark in accordance with the dark noise correction image.

2. The X-ray phase imaging apparatus according to claim 1.

3. a threshold setting unit that can arbitrarily set the threshold, the image generation unit selects the normal corrected image, the brightened corrected image, and / or the darkened corrected image using the threshold value, and combines the selected corrected images to generate one image of the subject; 3. The X-ray phase imaging apparatus according to claim 2.

4. An image processing method for generating an image of an object by an image processing unit, the method comprising: placing an object between an X-ray source and a detector; detecting X-rays irradiated onto the object from the X-ray source; and generating a plurality of noise correction images for brightness and darkness to correct luminance variations between pixels; irradiating the object with X-rays to generate a brightness image of the object; correcting the luminance image in accordance with the noise correction image to generate a plurality of corrected images of the subject; a step of selecting the plurality of corrected images using a threshold value to generate a single object image; An image processing method comprising:

5. In the step of generating a noise correction image, generating a normal noise correction image from a luminance image obtained by irradiating the X-rays without placing the subject, and generating a noise correction image brighter than the normal noise correction image and / or a noise correction image darker than the normal noise correction image using the luminance image and a standard deviation of luminance at each pixel calculated based on the luminance image; In the step of generating the corrected image, generating a normal corrected image corrected in accordance with the normal image for noise correction, a corrected image corrected to be bright in accordance with the bright image for noise correction, and / or a corrected image corrected to be dark in accordance with the dark image for noise correction; 5. The image processing method according to claim 4.

6. In the step of generating the subject image, selecting the normal corrected image, the brightened corrected image, and / or the darkened corrected image using the threshold value, and combining the selected corrected images to generate one image of the subject; 6. The image processing method according to claim 5.

Images