Method for reducing noise and method for reducing ring artifact

Abstract

To provide a method for reducing noise of a permeability image obtained from an X-ray transmission image (noise derived from optical system and changing with passage of time), and a method for reducing a noise-induced ring artifact from a reconstructed image.SOLUTION: Provided is a method for reducing the noise of a permeability image obtained from an X-ray transmission image. The method includes: a step 1 for obtaining a plurality of transmission images while there are no transmission images of a measurement object and there are no measurement objects; a step 2 for selecting a permeability image having a noise of transmission image closest to the noise of a transmission image of the measurement object from a plurality of transmission images while there are no measurement objects; and a step 3 for obtaining a permeability image in which noise is reduced from the transmission image of the measurement object, against the selected transmission image as a background.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for reducing noise and a method for reducing ring artifacts. [Background technology]

[0002] X-ray CT is a method in which a sample is placed between an X-ray source and a detector, and a tomographic image is obtained using a computer from multiple X-ray transmission images collected while the sample is rotated (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-258718 Summary of the Invention [Problem to be solved by the invention]

[0004] Normally, noise caused by the optical system can be removed by dividing it by the transmitted image (background) without a sample, but this cannot be fully removed if the noise pattern fluctuates over time.

[0005] The present invention aims to solve the above-mentioned problems and provide a method for reducing noise (noise that changes over time due to the optical system) in a transmittance image obtained from an X-ray transmission image, and a method for reducing ring artifacts caused by noise in a reconstructed image. [Means for solving the problem]

[0006] The present invention provides a method for reducing noise in a transmittance image obtained from an X-ray transmission image, comprising: Step 1 of obtaining a transmission image of a measurement object and a plurality of transmission images in the absence of the measurement object; Step 2: selecting a transmission image having noise closest to the noise of the transmission image of the measurement object from a plurality of transmission images in the absence of the measurement object; Step 3: obtaining a noise-reduced transmittance image from the transmission image of the object to be measured using the selected transmission image as a background. The present invention relates to a method for reducing noise, including: [Effects of the Invention]

[0007] According to the present invention, there is provided a method for reducing noise in a transmittance image obtained from an X-ray transmission image, the method comprising step 1 of obtaining a transmission image of a measurement object and a plurality of transmission images in the absence of the measurement object, step 2 of selecting, from the plurality of transmission images in the absence of the measurement object, a transmission image having a transmission image noise closest to the noise of the transmission image of the measurement object, and step 3 of obtaining a noise-reduced transmittance image from the transmission image of the measurement object using the selected transmission image as a background, thereby providing a method for reducing noise in a transmittance image obtained from an X-ray transmission image (noise that changes over time due to the optical system). [Brief explanation of the drawings]

[0008] [Figure 1] 1 is an example of a transmission image of a measurement object (sample). [Figure 2] 1 is an example of a transmittance image in which noise has been reduced from a transmission image of a measurement object (sample). [Figure 3] This figure shows a transmission image of a sample, and also schematically shows three partial regions (square areas) near the sample where pattern matching was performed. [Figure 4] FIG. 10 is a diagram showing a transmittance image in which stripe noise has been reduced. [Figure 5] FIG. 10 is a diagram showing a transmittance image in which stripe noise has been reduced, using a background (one transmission image without a sample) captured immediately before capturing the transmission image of the sample. [Figure 6] FIG. 10 is a diagram showing a transmittance image in which stripe noise has been reduced when pattern matching is performed on the entire image. [Figure 7] FIG. 5 is a diagram showing a reconstructed image in which ring-shaped artifacts have been reduced by performing spatial filtering on the sinogram created from the transmittance image of FIG. 4. [Figure 8] FIG. 10 is a diagram showing a reconstructed image created from a transmittance image obtained using a background (one transmission image without a sample) captured immediately before capturing a transmission image of the sample. [Figure 9] FIG. 10 is a diagram showing a reconstructed image in which ring-shaped artifacts have been reduced by applying spatial filtering processing to a sinogram created from a transmittance image obtained by performing pattern matching on the entire image. DETAILED DESCRIPTION OF THE INVENTION

[0009] The method for reducing noise in a transmittance image is a method for reducing noise in a transmittance image obtained from an X-ray transmission image (a transmission image obtained by X-rays), and includes step 1 of obtaining a transmission image of the object to be measured and a plurality of transmission images in the absence of the object to be measured, step 2 of selecting, from the plurality of transmission images in the absence of the object to be measured, a transmission image having a transmission image noise closest to the noise of the transmission image of the object to be measured, and step 3 of obtaining a noise-reduced transmittance image from the transmission image of the object to be measured using the selected transmission image as a background (i.e., step 3 of reducing the noise in the transmittance image by dividing the transmission image of the object to be measured by using the selected transmission image as a background).

[0010] Usually, the background (transmitted image without a sample) is taken at a time close to the measurement time of the sample (object to be measured), but since stripe noise changes over time, a time close to the measurement time is not necessarily optimal. In contrast, the method for reducing noise in a transmittance image involves measuring the background multiple times, preferably as many times as possible, during the experimental period (measurement period) and selecting the pattern that most closely resembles the stripe pattern (striped noise) of the sample. The selected transmission image with the most similar striped noise is then used as the background, and the transmission image of the sample is divided by the background to reduce or remove noise from the transmission image, thereby obtaining a transmittance image of the sample with reduced or removed striped noise.

[0011] <Method for reducing noise in transmittance images> Hereinafter, a method for reducing noise in the transmittance image will be described in more detail, but the method is not limited to the following method.

[0012] The method for reducing noise in the transmittance image is a method for reducing noise in a transmittance image obtained from an X-ray transmission image (a transmission image obtained by X-rays), and first, step 1 is performed in which a transmission image of a measurement object (sample) and a plurality of transmission images in the absence of the measurement object (sample) are obtained.

[0013] In step 1, the transmission image of the sample can be obtained by taking a transmission image of the sample using a known device capable of taking a transmission image using X-rays. Figure 1 shows an example of a transmission image of a sample. It can be seen that the transmission image in Figure 1 contains striped noise.

[0014] In the above step 1, a plurality of transmission images in the absence of a sample can also be obtained by taking a plurality of transmission images in the absence of a sample using a known device. In this case, in step 2 described below, the transmission image that is closest to the noise pattern of the transmission image of the sample will be selected from among these plurality of transmission images, so from the viewpoint of obtaining a transmission image with a noise pattern that is as close as possible, it is desirable to obtain as many transmission images in the absence of a sample as possible.

[0015] In the method for reducing noise in the transmittance image, after obtaining a transmission image of the object to be measured (sample) and a plurality of transmission images in the absence of the object to be measured (sample) in step 1, step 2 is performed in which a transmission image having transmission image noise closest to the noise of the transmission image of the object to be measured (sample) is selected from the plurality of transmission images obtained in the absence of the object to be measured (sample).

[0016] A known pattern matching technique can be used to select, from among a plurality of transmission images taken in the absence of a sample, a transmission image having noise closest to that of the transmission image of the sample. As pattern matching, various techniques can be used, including template matching and a technique of extracting feature values ​​and performing vector matching.

[0017] Among these, it is preferable to use phase-only correlation to perform pattern matching, from the viewpoint of being able to suitably perform pattern matching between the sample and background stripe noise. Phase-only correlation is a well-known technique, and is described, for example, in "High-Precision Machine Vision Based on Phase-Only Correlation - Aiming for Image Sensing Technology that Breaks the Barrier of Pixel Resolution" (Fundamentals Review 1.1 (2007): pp. 30-40, Institute of Electronics, Information and Communication Engineers, Fundamentals and Boundaries Society, Aoki, Takafumi et al.).

[0018] In the above step 2, if pattern matching is performed using a phase-only correlation method or the like and the contrast of the striped noise is low and matching is not successful, it is preferable to use a Log filter from the viewpoint of extracting the contours of the stripes. The Log filter is well known and refers to a filtering process in which an image is smoothed using a Gaussian filter to reduce noise, and then a Laplacian filter is used to detect the contours.

[0019] In step 2 above, the way stripes change is not uniform within the projected image and can vary greatly depending on the location. Therefore, when stripe pattern matching is performed on the entire image, some stripes within the projected image can be removed, but strong stripes often remain in other locations. On the other hand, even if some stripes remain after dividing the background, it is possible to significantly reduce the occurrence of artifacts by performing spatial filtering or other processing later. Therefore, a background in which strong stripes are eliminated, even if some stripes remain, is more preferable than a background in which strong stripes remain even though some stripes are removed.

[0020] For this reason, in step 2 above, it is more desirable to perform pattern matching in a partial region of the transmission image near the sample (object to be measured) rather than performing pattern matching in the entire region of the transmission image. When performing pattern matching in a partial region of the transmission image near the sample, it is desirable not only to minimize the deviation of the stripes between the stripe noise in the transmission image of the sample and the stripe noise in the transmission image without the sample, but also to make the degree of deviation approximately the same. Pattern matching in a partial region of the transmission image near the sample (object to be measured) is preferably performed in approximately 3 to 5 partial regions within the transmission image.

[0021] In step 2 above, to match the stripes, it is necessary to shift the transmission image without the measurement target (sample). It is desirable to use the average of the shifts determined by pattern matching for the partial regions as the shift amount for pattern matching. For example, if a shift of +5 is optimal for the upper part of the image and a shift of +3 is optimal for the lower part, setting the shift amount to the average of 4 will result in a reasonably good match, though not the best. Furthermore, achieving a moderate match can suppress the occurrence of artifacts through spatial filtering, as described above. This allows prioritizing the elimination of artifacts in the reconstructed image over reducing noise in the transmission image.

[0022] In step 2, it is desirable to determine the deviation between the transmission image of the object to be measured (sample) and the transmission image in the absence of the object to be measured (sample) in the partial region after being moved by the above-mentioned movement amount (shift amount), and to select the transmission image with the smallest deviation from the multiple transmission images in the absence of the object to be measured. When pattern matching is performed in two or more partial regions (e.g., 3 to 5 locations), pattern matching is usually performed in each partial region before selection.

[0023] The method for reducing noise in a transmittance image further includes step 3 of obtaining a transmittance image with reduced noise from the transmission image of the object to be measured (sample) using as a background a transmission image with transmission image noise closest to the noise of the transmission image of the object to be measured (sample) selected in step 2 above (i.e., step 3 of reducing noise in the transmittance image by dividing the transmission image of the object to be measured by using the selected transmission image as a background). By step 3 above, a transmittance image with reduced or removed noise can be obtained from the transmission image of the sample.

[0024] Figure 2 shows an example of a transmittance image in which noise has been reduced from a transmission image of a sample. It can be seen that the transmittance image in Figure 2 has less stripe noise than the transmission image in Figure 1.

[0025] <Methods for reducing ring artifacts> The method for reducing the ring artifact is a method in which, after step 3, step 4 is performed to reduce the ring artifact by applying spatial filtering processing to a sinogram created from a transmittance image.

[0026] In the method for reducing ring artifacts described above, if it is difficult to completely remove the stripe noise in step 3, the ring artifacts can be reduced by applying spatial filtering to the sinogram created from the transmittance image of the sample obtained in step 3. Therefore, it is possible to obtain a reconstructed image of the sample in which the ring artifacts have been reduced or removed.

[0027] The spatial filtering process is well known and is a method described in, for example, Beat Munch, et al., Opt. Express 17(10) 8567-8591 (2009).

[0028] The measurement object (sample) applicable to the method for reducing noise in the transmission image and the method for reducing ring artifacts is not particularly limited, and examples thereof include any sample from which an X-ray transmission image can be obtained using a known device, and any sample from which a reconstructed image can be obtained using a known X-ray CT device.

[0029] Specific examples of such measurement objects (samples) include samples of rubber components alone, and samples of rubber compositions containing rubber components and other components.

[0030] The rubber component is not particularly limited, and for example, diene rubber can be used. Examples of diene rubber include isoprene rubber, butadiene rubber (BR), styrene butadiene rubber (SBR), styrene isoprene butadiene rubber (SIBR), ethylene propylene diene rubber (EPDM), chloroprene rubber (CR), and acrylonitrile butadiene rubber (NBR). Other examples include butyl rubber and fluororubber. These may be used alone or in combination of two or more.

[0031] The rubber composition may include the rubber component and a filler, such as silica, carbon black, calcium carbonate, talc, alumina, clay, aluminum hydroxide, aluminum oxide, and mica. For example, the method can be suitably applied to a rubber composition sample containing silica and / or carbon black.

[0032] The rubber composition may contain other components in addition to the rubber component and filler. Specifically, the present invention is suitable for use with rubber composition samples containing at least one compounding agent commonly used in the tire industry, such as a plasticizer, an antioxidant, stearic acid, wax, zinc oxide, sulfur, or a vulcanization accelerator.

[0033] A sample of the rubber component alone and a sample of a rubber composition containing the rubber component and other components can be produced by a common method, for example, by kneading the compounding materials in a kneader such as a Banbury mixer or an open roll mixer, followed by vulcanization as necessary. [Example]

[0034] Below, examples (embodiments) that are considered preferable for implementation will be shown, but the scope of the present invention is not limited to the examples.

[0035] The various chemicals used in tire manufacturing are summarized below. If necessary, the chemicals may be refined according to standard methods. NR:TSR20 BR: BR150B (cis content 97% by mass) manufactured by Ube Industries, Ltd. Carbon black: Diablack N220 (Mitsubishi Chemical Corporation, N2SA114m 2 / g) Stearic acid: NOF Corporation's "Tsubaki" stearic acid Zinc oxide: Zinc oxide No. 1 manufactured by Mitsui Mining & Smelting Co., Ltd. Sulfur: HK-200-5 (powdered sulfur) manufactured by Hosoi Chemical Industry Co., Ltd. Vulcanization accelerator: Noccela CZ (N-cyclohexyl-2-benzothiazolyl sulfenamide) manufactured by Ouchi Shinko Chemical Industry Co., Ltd.

[0036] <Preparation of vulcanized rubber composition (sample)> According to the formulation shown in Table 1, materials other than sulfur and vulcanization accelerator were kneaded for 5 minutes at 150°C using a 1.7 L Banbury mixer manufactured by Kobe Steel, Ltd. to obtain a kneaded mixture. Sulfur and a vulcanization accelerator were added to the kneaded material, and kneading was carried out for 5 minutes under the condition of 80 °C using an open roll to obtain an unvulcanized rubber composition. The unvulcanized rubber composition was vulcanized at 170 °C for 12 minutes to obtain a vulcanized rubber composition (sample).

[0037]

Table 1

[0038] Regarding the vulcanized rubber composition (sample) in Table 1, the following "creation of transmittance image" and "creation of reconstructed image" were carried out using the following X-ray CT apparatus. <X-ray CT apparatus> Measurement was carried out under the same measurement conditions using an apparatus equivalent to that described in Japanese Patent Application Laid-Open No. 2022-095400.

[0039] <Creation of transmittance image> (Step 1) Using the X-ray CT apparatus, a transmittance image (1 sheet) of the above sample and transmittance images (142 sheets) in the state where the above sample was absent were obtained. Fig. 3 shows the transmittance image of the above sample. (Step 2) Pattern matching by the phase-limited correlation method was carried out using the transmittance images (142 sheets) in the state where the above sample was absent. When the contrast of the stripes was low and good matching could not be achieved, the contour of the stripes was extracted using a Log filter. The stripe pattern matching was performed not on the entire image but on partial regions (three partial regions). Fig. 3 shows three partial regions (square portions) near the sample where the pattern matching was performed. <U The amount of movement (shift amount) when moving the image to match the stripes was the average value of the amounts of movement obtained by the pattern matching of the partial regions. The deviation in each partial region after moving with the above shift amount was obtained, and the background (transmittance image in the state where the above sample was absent) with the smallest deviation was adopted. (Step 3) Using the above background, a transmittance image with reduced stripe noise was obtained from the transmission image of the sample. Figure 4 shows the transmittance image with reduced stripe noise obtained by the above method.

[0040] On the other hand, Figure 5 shows a transmittance image in which stripe noise has been reduced using a background image (one transmission image without a sample) taken immediately before the above-mentioned sample transmission image was taken. It can be seen that stripe noise is reduced more in Figure 4 than in Figure 5.

[0041] Figure 6 shows a transmittance image in which stripe noise has been reduced when pattern matching is performed on the entire image rather than on a partial region in the above-mentioned <Creating a transmittance image>. It can be seen that stripe noise is reduced in Figure 6 compared to Figure 5, while stripe noise was reduced even more in Figure 4, in which pattern matching was performed on a partial region.

[0042] <Creating a reconstruction image> (Step 4) A sinogram was created from the transmittance image of the sample obtained in the above <Creating a transmittance image>, and then spatial filtering was applied to the created sinogram to further reduce noise before reconstruction, resulting in a reconstructed image with reduced ring-shaped artifacts. Figure 7 shows a reconstructed image with reduced ring-shaped artifacts using the above method.

[0043] On the other hand, Figure 8 shows a reconstructed image created from a transmittance image obtained using a background image (one transmission image without a sample) captured immediately before the above-mentioned transmission image of the sample was captured. It can be seen that the ring-shaped artifacts are reduced in Figure 7 compared to Figure 8.

[0044] Figure 9 shows a reconstructed image in which ring-shaped artifacts have been reduced by applying spatial filtering to a sinogram created from a transmittance image obtained by pattern matching on the entire image, rather than on a partial region, in the above-mentioned <Creating a reconstructed image>. It can be seen that stripe noise has been reduced in Figure 9 compared to Figure 8, while ring-shaped artifacts have been further reduced in Figure 7, in which pattern matching was performed on a partial region.

[0045] The present invention (1) is a method for reducing noise in a transmittance image obtained from an X-ray transmission image, comprising: Step 1 of obtaining a transmission image of a measurement object and a plurality of transmission images in the absence of the measurement object; Step 2: selecting a transmission image having noise closest to the noise of the transmission image of the measurement object from a plurality of transmission images in the absence of the measurement object; Step 3: obtaining a noise-reduced transmittance image from the transmission image of the object to be measured using the selected transmission image as a background. This is a method for reducing noise including:

[0046] The present invention (2) is the method for reducing noise according to the present invention (1), in which step 2 is selection using pattern matching.

[0047] The present invention (3) is a method for reducing noise according to the present invention (2), in which pattern matching is performed in a partial region within a transmission image.

[0048] The present invention (4) is a method for reducing noise according to the present invention (3), in which the average value of the movement amounts determined by pattern matching of partial regions is used as the movement amount for pattern matching.

[0049] The present invention (5) is a method for reducing noise according to the present invention (4), which determines the deviation between a transmission image of the object to be measured and a plurality of transmission images in the absence of the object to be measured in a partial region, and selects the transmission image with the smallest deviation from the plurality of transmission images in the absence of the object to be measured.

[0050] The present invention (6) is a method for reducing ring artifacts, which performs step 4 of reducing ring artifacts by applying spatial filtering processing to a sinogram created from a transmittance image after step 3.

Claims

1. A method for reducing noise in a transmittance image obtained from an X-ray transmission image, comprising: Step 1: obtaining a transmission image of a measurement object and a plurality of transmission images in the absence of the measurement object; Step 2: selecting a transmission image having a transmission image noise closest to the noise of the transmission image of the measurement object from a plurality of transmission images in a state where the measurement object is not present; Step 3: obtaining a noise-reduced transmittance image from the transmission image of the object to be measured using the selected transmission image as a background; A method for reducing noise, including:

2. 2. The method for reducing noise according to claim 1, wherein step 2 selects using pattern matching.

3. 3. The method for reducing noise according to claim 2, wherein pattern matching is performed in a partial region within the transmission image.

4. 4. The method for reducing noise according to claim 3, wherein an average value of the amounts of movement determined by pattern matching of partial regions is used as the amount of movement in pattern matching.

5. 5. The method for reducing noise according to claim 4, further comprising: determining, in a partial region, a deviation between a transmission image of the object to be measured and a plurality of transmission images in a state in which the object to be measured is not present; and selecting, from the plurality of transmission images in a state in which the object to be measured is not present, the transmission image in which the deviation is smallest.

6. After step 3, the method for reducing ring artifacts includes step 4 of reducing ring artifacts by applying spatial filtering to a sinogram created from the transmittance image.

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