Thickness detection method, pipe inspection method, and reference piece

Abstract

The present invention provides a thickness detection method, a pipe inspection method, and a reference piece that reduce X-ray scattering and pixel density disturbances in transmitted images caused by the reference piece, and enable the acquisition of thickness information from transmitted images more continuously and stably. [Solution] Laser light is shone from a laser light irradiator 11 onto IP2 on which an image of the piping is recorded, and the resulting exhaust emission is read by a light receiving unit 112 and a photoelectric conversion unit 113. Based on the reading results, an image generation unit 114 generates an X-ray transmission image showing the piping. PC120 derives the thickness of the thinned portion of the piping based on the X-ray transmission image generated by the image generation unit 114. The formula logY=P*logX+Q is used to derive the thickness, where X is the thickness of the thinned portion and Y is the pixel value of the pixel in the X-ray transmission image. Instead of piping, X-rays are shone onto a reference piece located on the reference irradiation surface, which has a reference irradiation surface in which the thickness changes continuously in the direction of X-ray irradiation, and each of the first to n reference pieces has a thickness of X1 to Xn (n: an integer of 2 or more), and the constants P and Q are derived.

Description

[[Technical Field]]

[0001] The present invention relates to a thickness detection method, a piping inspection method, and a reference piece. [[Background Art]]

[0002] Conventionally, as disclosed in Patent Document 1, there is a technology in which X-rays are irradiated onto a detection target object such as a pipe, an X-ray film is exposed by X-rays transmitted through the detection target object (hereinafter referred to as transmitted X-rays), and the thickness of the detection target object is measured from the density of pixels included in an image formed on the X-ray film. On the other hand, as a technology for obtaining an image of a detection target object by transmitted X-rays, there is also a method using a plate holding a photostimulable phosphor (imaging plate), as disclosed in Patent Document 2. According to Patent Document 2, after irradiating an imaging plate with transmitted X-rays, the imaging plate is irradiated with excitation light to cause the photostimulable phosphor to emit photostimulated luminescence, and the luminescence is detected to read an image of the detection target object recorded on the imaging plate.

[0003] When using an X-ray film, as disclosed in Patent Document 1, attempts have been made to easily and quickly detect the thickness of a detection target object from an image on the X-ray film by using a known relationship between the thickness of the detection target object and the pixel density. Similarly, there is a demand for a method that can easily and quickly detect the thickness of a detection target object from an image obtained using an imaging plate.

[0004] For example, Patent Document 3 discloses a thickness detection method that can detect the thickness of an object to be detected from an image acquired using a plate holding a photostimulable phosphor, and a pipe inspection method that applies this method. The thickness detection method disclosed in Patent Document 3 comprises: an irradiation step of irradiating a photostimulable phosphor held on a plate with X-rays that have passed through the object to be detected; an image generation step of reading the image of the object to be detected recorded on the photostimulable phosphor of the plate in the irradiation step and generating an X-ray transmission image showing the object to be detected; and a thickness derivation step of deriving the thickness of the object to be detected based on a predetermined relationship that X and Y satisfy, where X is the thickness of the object to be detected and Y is the pixel value of the pixels in the X-ray transmission image generated in the image generation step, and the predetermined relationship is logY = P * logX + Q or an equivalent formula, where P and Q are constants. The thickness detection method represented by the formula includes, instead of the object to be detected, a reference derivation step in which two steps, an irradiation step and an image generation step, are performed on first to n reference pieces having thicknesses Y1 to Yn (n: an integer of 2 or more), and when Xi (i: an integer of 1 or more and less than or equal to n) is the pixel value of the i-th reference piece, P and Q are derived such that (X,Y)=(Xi,Yi) satisfies or approximately satisfies a predetermined relationship for i=1,2,...n. The main detection step in which two steps are performed on the object to be detected and the thickness of the object to be detected is detected based on a predetermined relationship using P and Q derived in the reference derivation step.

[0005] However, while the reference piece disclosed in Patent Document 3 is constructed by connecting plate-shaped members of different thicknesses in a stepped manner, conventional stepped reference pieces have a structure in which the thickness varies discretely. As shown in the profile (pixel value profile) 500 in Figures 17 and 18, X-ray scattering and edge effects are significantly observed at the stepped portions, resulting in the problem of unwanted density fluctuations of pixels in the transmitted image. Therefore, it becomes necessary for evaluators to visually select areas with less scattering effect, which can easily lead to variability in evaluation results. Additionally, there is the problem of increased workload due to the need to find the optimal measurement location from the reference piece and the need for calibration work. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2002-267433 [Patent Document 2] International Publication No. 2016 / 136296 [Patent Document 3] Japanese Patent Publication No. 2020-118552 [Overview of the project] [Problems that the invention aims to solve]

[0007] Therefore, the present invention aims to provide a thickness detection method, a pipe inspection method, and a reference piece that reduce X-ray scattering and pixel density disturbances in transmitted images caused by the reference piece, and enable the acquisition of thickness information from transmitted images more continuously and stably. [Means for solving the problem]

[0008] To solve the above problems, the present invention provides a thickness detection method comprising: an irradiation step of irradiating a photostimulable phosphor held on a plate with X-rays transmitted through the object to be detected; an image generation step of reading the image of the object to be detected recorded on the photostimulable phosphor of the plate in the irradiation step and generating an X-ray transmission image showing the object to be detected; and a thickness derivation step of deriving the thickness of the object to be detected based on a predetermined relationship that satisfies X and Y, where X is the thickness of the object to be detected and Y is the pixel value of the pixel in the X-ray transmission image generated in the image generation step, wherein the predetermined relationship is expressed as logY = P * logX + Q or an equivalent formula, where P and Q are constants, and the detection Instead of a target object, the system includes a reference piece having a reference irradiation surface in which the thickness in the direction of X-ray irradiation changes continuously, and where the first to nth reference parts, each having a thickness of X1 to Xn (n: an integer of 2 or more), are located on the reference irradiation surface. The system performs two steps, an irradiation step and an image generation step, on a reference piece, and derives P and Q such that (X,Y)=(Xi,Yi) satisfies or approximately satisfies a predetermined relationship for i=1,2,...n, when Yi (i: an integer of 1 or more and less than or equal to n) is the pixel value relating to the ith reference part. The system also includes a reference derivation step, which performs two steps on a target object, and a main detection step, which uses the P and Q derived in the reference derivation step to detect the thickness of the target object based on a predetermined relationship.

[0009] As a reference specimen, a specimen with a wedge-shaped cross-section along the direction of X-ray irradiation can be used.

[0010] A position that is a certain distance or more from the edge of the reference irradiation surface can be used as the reference area.

[0011] Furthermore, the above thickness detection method can be used to create a pipe inspection method for detecting the thickness of a pipe as the object to be detected.

[0012] The detection process includes a graph output step in which an output means outputs a graph showing the distribution of pixel values ​​on a virtual line passing through the thinned portion of the pipe or its vicinity along an X-ray transmission image, and a sound thickness estimation step in which, based on the graph output by the output means, the estimated thickness of the thinned portion assuming that no thinning had occurred is derived, and the thickness of the thinned portion can be evaluated against the estimated thickness in this detection step.

[0013] The estimated thickness can be derived based on the pixel value at a point on the interpolation curve that shows the shape of the graph at the location corresponding to the thinned area, assuming that no thinning occurred, and a predetermined relationship.

[0014] In the graph output process, a graph is output to the output means for each of two different virtual lines, and in the sound thickness estimation process, the thickness of the sound area can be estimated based on the two graphs relating to the two virtual lines.

[0015] Furthermore, the reference piece according to the present invention is used to derive P and Q such that, when X is the thickness of the object to be detected and Y is the pixel value of the pixels in the X-ray transmission image, X and Y satisfy or approximately satisfy a predetermined relationship expressed as logY = P * logX + Q or an equivalent formula, where P and Q are constants, and the reference piece has a reference irradiation surface in which the thickness in the direction of X-ray irradiation changes continuously, and the first to n reference parts having thicknesses X1 to Xn (n: an integer of 2 or more) are located on the reference irradiation surface.

[0016] The cross-section along the direction of X-ray irradiation can be made wedge-shaped.

[0017] The reference section can be spaced a certain distance or more from the edge of the reference irradiation surface.

[0018] Further, the reference piece according to the present invention is a reference piece used for deriving a mathematical expression representing a predetermined relationship that X and Y satisfy or approximately satisfy, where X is the thickness of an object to be detected, and Y is the pixel value of a pixel in an X-ray transmission image generated by reading an image of the object to be detected recorded on a photostimulable phosphor irradiated with X-rays transmitted through the object to be detected, wherein the reference piece has a reference irradiation surface whose thickness in the X-ray irradiation direction continuously changes, and first to n-th reference portions respectively having thicknesses X1 to Xn (n is an integer of 2 or more) are located on the reference irradiation surface.

[0019] A cross-section along the X-ray irradiation direction may be wedge-shaped.

[0020] The reference portion may be spaced apart from an end of the reference irradiation surface by a predetermined distance or more.

Effects of the Invention

[0021] According to the present invention, there can be provided a thickness detection method, a piping inspection method, and a reference piece that reduce X-ray scattering caused by the reference piece and density disturbance of pixels in a transmission image, and can obtain thickness information more continuously and stably from the transmission image.

Brief Description of Drawings

[0022] [Figure 1] It is a schematic diagram of a step of recording X-ray transmission images of a pipe and a reference piece on an IP in the pipe inspection method according to an embodiment of the present invention. [Figure 2] They are a bottom view (a) and a front view (b) of the reference piece used in the step of FIG. 1. [Figure 3] It is a functional block diagram showing a configuration of an image analysis system that performs image analysis based on an IP on which an X-ray transmission image is recorded in the step of FIG. 1. [Figure 4] It is a flow chart showing a series of flows from recording an X-ray transmission image to deriving a wall thickness reduction rate of a pipe. [Figure 5] It is an example of an X-ray transmission image. [Figure 6] It is a graph showing a distribution of pixel values on a virtual line having two points on the X-ray transmission image of FIG. 5 as both ends. [Figure 7] This graph shows the distribution of pixel values ​​along a virtual line with two other points at its ends on the X-ray transmission image in Figure 5. [Figure 8] This is a schematic diagram showing the relationship between the X-ray transmission direction and the radial direction of the piping. [Figure 9] This is an example of a profile of a reference specimen according to one embodiment of the present invention. [Figure 10] This is an example of a profile of a reference specimen according to one embodiment of the present invention. [Figure 11] This is an example of a profile of a reference specimen according to one embodiment of the present invention. [Figure 12] Figure 1 shows a bottom view (a) and a front view (b) of a modified example of the standard piece used in the process shown in Figure 1. [Figure 13] Figure 1 shows a bottom view (a) and a front view (b) of a modified example of the standard piece used in the process shown in Figure 1. [Figure 14] Figure 1 shows a bottom view (a) and a front view (b) of a modified example of the standard piece used in the process shown in Figure 1. [Figure 15] This is a schematic diagram of the process for recording X-ray transmission images of pipes and reference pieces on an IP in a conventional pipe inspection method. [Figure 16] These are a bottom view (a) and a front view (b) of a reference piece used in a conventional pipe inspection method. [Figure 17] This is an example of a profile of a reference piece according to a conventional invention. [Figure 18] This is an example of a profile of a reference piece according to a conventional invention. [Modes for carrying out the invention]

[0023] A pipe inspection method according to one embodiment of the present invention will be described with reference to Figures 1 to 8. This pipe inspection method corresponds to both the thickness detection method and the pipe inspection method in the present invention. In each figure, components with the same reference numerals are the same or equivalent.

[0024] The pipe inspection method according to this embodiment relates to a method for inspecting the corrosion status of individual pipes included in piping systems constructed in public facilities such as office buildings and government buildings, schools, hospitals, hotels, apartment buildings, private residences, swimming pools, etc. The pipes to be inspected (hereinafter referred to as "target pipes 3") are installed in various locations within the piping system. Target pipes 3 are any of the following: sanitary pipes (water supply pipes, hot water pipes, wastewater pipes, sewage pipes, rainwater pipes, fire extinguishing pipes, sprinkler pipes, etc.) having a cylindrical body; air conditioning pipes (chilled water pipes, hot water pipes, cooling water pipes, refrigerant pipes, drain pipes, etc.); and water and sewage pipes. Furthermore, target pipes 3 are made of any of the following: steel pipes (carbon steel pipes, etc.), cast iron pipes, lined steel pipes, copper pipes, stainless steel pipes, etc. This pipe inspection method uses an X-ray generator 1, an imaging plate (hereinafter referred to as "IP") 2, and a reference piece 4 as shown in Figure 1. The X-ray generator 1 is a device that irradiates objects such as target pipes 3 and reference pieces 4 with X-rays. Corrosion in target pipe 3 manifests as thinning of the wall, which is a reduction in the thickness of the wall. Therefore, the degree of corrosion in target pipe 3 can be evaluated by assessing how much the thickness has decreased compared to the sound sections where no corrosion is present.

[0025] The reference piece 4 is a reference object (reference object) used to determine the thickness of the target pipe 3. As shown in Figure 2, the reference piece 4 is a metal member having a reference irradiation surface 5 in which the wall thickness in the direction of X-ray irradiation changes continuously. Because the wall thickness in the direction of X-ray irradiation changes continuously, it is possible to prevent X-rays from passing through discontinuous steps, thereby suppressing the occurrence of scattering. In this embodiment, the reference piece 4 is configured such that the cross section along the direction of X-ray irradiation is wedge-shaped. The reference piece 4 is made of a material appropriate to the material of the target pipe 3. For example, reference pieces 4 made of steel, stainless steel, and copper can be used, respectively. The reference piece 4 is provided with reference sections 4A to 4E, each with a different wall thickness in the direction of X-ray irradiation, located on the reference irradiation surface 5. The positions of the reference sections 4A to 4E can be indicated and specified by marking them on the reference irradiation surface 5. Alternatively, the positions of the reference sections 4A to 4E may be specified, for example, by the distance from a predetermined end. By positioning the reference sections 4A to 4E at a certain distance from the edges of the reference irradiation surface 5, the effects of scattering and other factors at the edges can be avoided. The thickness of the X-ray irradiation direction in the reference sections 4A to 4E has been measured in advance.

[0026] The imaging plate 2 comprises a light-shielding synthetic resin plate and a photostimulable phosphor layer formed on the entire surface of the plate. The photostimulable phosphor layer is a layer containing photostimulable phosphor. X-rays from the X-ray generator 1 that have passed through the target pipe 3 or reference piece 4 (hereinafter referred to as transmitted X-rays) and X-rays that have propagated from the X-ray generator 1 without passing through the object (hereinafter referred to as radiopaque X-rays) are irradiated onto the surface of IP2 on the photostimulable phosphor layer side (hereinafter referred to as the irradiation surface). The X-ray irradiation time is adjusted so that the dose on the back side of the object to be detected (the side opposite to the X-ray generator 1) is at a predetermined level (for example, 1000 μSv). The transmitted X-rays irradiated onto IP2 have a lower intensity as the thickness of the target pipe 3 and reference piece 4 increases, due to absorption and reflection by the target pipe 3 and reference piece 4. On the other hand, radiopaque X-rays are irradiated onto IP2 without being absorbed by the object. As a result, images of the target pipe 3 and the reference piece 4 (hereinafter referred to as the X-ray transmission image) are recorded as latent images in the photostimulable phosphor layer of IP2. In this embodiment, unless otherwise specified, the portion of the target pipe 3 that is approximately directly in front of the X-ray generator 1, that is, the portion that can be considered to be aligned in a direction approximately perpendicular to the imaging plate 2, is the target of inspection.

[0027] The IP2 on which the X-ray transmission image is recorded is analyzed using the image analysis system 100 shown in Figure 3. The image analysis system 100 comprises an IP reader 110, a computer (hereinafter referred to as PC) 120, and a display 131 and an input device 132. The IP reader 110 has a laser light irradiator 111, a light receiving unit 112, a photoelectric conversion unit 113, and an image generation unit 114. The laser light irradiator 111 excites the phototextative phosphor in the phototextative phosphor layer by irradiating the irradiation surface of the IP2 with laser light, thereby generating phototextative emission in the phototextative phosphor layer. The light receiving unit 112 has an optical system consisting of a lens or the like that receives the phototextative emission. The photoelectric conversion unit 113 photoelectrically converts the phototextative emission received by the light receiving unit 112 into a current corresponding to its intensity. The image generation unit 114 generates image data corresponding to the X-ray transmission image based on the current generated by the photoelectric conversion unit 113. This image data consists of data indicating the pixel value of each pixel constituting the X-ray transmission image. Specifically, the image data consists of data indicating the pixel values ​​of pixels arranged in both the vertical and horizontal directions. Each pixel value has a magnitude corresponding to the intensity of exhausted emission generated from the region corresponding to each pixel on the irradiation surface of IP2. In this embodiment, a larger pixel value indicates a greater intensity of exhausted emission. The intensity of exhausted emission is greater with increasing exposure to transmitted X-rays. Therefore, a larger pixel value indicates a greater intensity of transmitted X-rays, i.e., a smaller thickness of the object.

[0028] The PC120 comprises hardware such as a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), and a hard disk, as well as software consisting of program data stored in memory units such as ROM and RAM. The following functions of the PC120 are realized through the cooperation of this hardware and software. The PC120 is connected to a display 131. The results of various information processing performed by the PC120 are displayed on the screen of the display 131. The PC120 is also connected to input devices 132, such as a keyboard and a pointing device, and accepts user input using these devices.

[0029] Image data showing an X-ray transmission image is transmitted to the PC120 from the IP reader 110. An image analysis application and a thickness derivation application are installed on the PC120. The image analysis application displays various images, such as the X-ray transmission image, on the display 131 based on the image data, and displays characteristic values ​​related to a specific range in the X-ray transmission image on the display 131 based on user input via the input device 132. The thickness derivation application also causes the PC120 to function to derive the thickness of the thinned portion of the target pipe 3 and the estimated sound thickness, as described later, based on user input of analysis values ​​obtained as a result of the image analysis.

[0030] The following describes the process from recording an X-ray transmission image onto IP2 to deriving the wall thinning rate of the target pipe 3, with reference to Figure 4. First, as shown in Figure 1, X-rays from the X-ray generator 1 are irradiated onto IP2 while passing through the target pipe 3 and the reference piece 4 (Step S1 in Figure 4; irradiation process in the present invention). This records an X-ray transmission image of the target pipe 3 and the reference piece 4 onto IP2. Next, the IP reader 110 reads the recorded image on IP2 by photoelectric conversion of the exhausted emission caused by laser light irradiation (Step S2). Then, the IP reader 110 generates image data corresponding to the X-ray transmission image based on the current generated by the photoelectric conversion, which corresponds to the intensity of the exhausted emission (Step S3). Steps S2 and S3 correspond to the image generation process in the present invention.

[0031] Next, using the functions of the PC120 provided by the image analysis application and the thickness derivation application, image analysis based on the image data generated by the IP reader 110 and the derivation of the thickness of the thinned and sound sections are performed as follows (steps S4 to S12). First, the PC120 displays an X-ray transmission image on the screen of the display 131 (step S4). An example of the X-ray transmission image shown in Figure 5 includes images of the target pipe 3 and the reference piece 4. The image of the target pipe 3 includes sound sections that appear relatively white and multiple thinned sections 3x that appear darker than the surrounding areas. Note that the X-ray transmission image in Figure 5 was obtained using a target pipe 3 in which multiple round recesses, representing actual thinned sections due to corrosion, were formed on the pipe wall by metalworking, but the following explanation is also applicable when dealing with actual pipes in which thinned sections have been formed due to corrosion.

[0032] Next, the PC120 derives the diameter of the target pipe 3 by deriving the distance between one end and the other end of the target pipe 3 on the X-ray transmission image (step S5). The one end and the other end of the target pipe 3 are specified based on user input from the input device 132. The crosses P1 and P2 in Figure 5 are an example of cursor images indicating the positions of the one end and the other end specified in this way. Next, representative pixel values ​​are derived for each of the reference sections 4A to 4E of the reference piece 4 (step S6). ROI (Region of Interest) analysis performed by the PC120 is used to derive the representative pixel values. In the ROI analysis, characteristic values ​​for pixels within the ROI set based on user input from the input device 132 are derived and displayed on the screen of the display 131. In Figure 5, as an example, characteristic values ​​are displayed for ROI R1 set centered on reference section 4C. In this example, the characteristic values ​​include the minimum pixel value, maximum pixel value, average pixel value, standard deviation, mode pixel value, and mode number of pixels within the ROI. Based on this, for example, the average pixel value of 2667 is derived as the representative pixel value of the reference unit 4C. Other values, such as the mode, may also be derived as the representative pixel value. Furthermore, if the positions of the reference units 4A to 4E are determined by, for example, the distance from a predetermined end, in step S6, the PC 120 may identify the area to be analyzed one-dimensionally (linearly) or two-dimensionally (planarly) without requiring user input from the input device 132, and derive the representative pixel value.

[0033] Next, the PC120 derives the distance from one end of the target pipe 3 to the thinned section 3x on the X-ray transmission image (step S7). The positions of one end of the target pipe 3 and the thinned section 3x are specified based on user input from the input device 132. The crosses P3 and P4 in Figure 5 are examples of cursor images showing the positions of the end and the thinned section 3x specified in this way. Next, a representative pixel value for the thinned section 3x of the target pipe 3 is derived (step S8). Similar to step S6, ROI analysis is used to derive the representative pixel value. The maximum pixel value within the ROI set to include the thinned section 3x is derived as the representative value. As an example, Figure 5 shows characteristic values ​​for ROI R2 set to include only one thinned section 3x. Based on this, the maximum pixel value of 2737 is derived as the representative pixel value for the thinned section 3x.

[0034] Next, the pixel values ​​(hereinafter referred to as estimated healthy pixel values) are estimated assuming that no thinning occurred in the thinned section 3x of the target pipe 3 (step S9). The estimated healthy pixel values ​​are obtained in order to derive the thickness of the thinned section 3x if there was no thinning (hereinafter referred to as estimated healthy thickness). The estimated healthy pixel values ​​are derived based on user input made using the input device 132 while referring to the distribution of pixel values ​​on a virtual line passing through the thinned section 3x. Preferably, at least two distributions of pixel values ​​are used: one relating to a virtual line that is approximately along the length direction of the target pipe 3, and another relating to a virtual line that intersects the length direction of the target pipe 3 (for example, a direction perpendicular to the length direction of the target pipe 3). ROI R3 and R4 in Figure 5 are examples of regions corresponding to such virtual lines. ROI R3 is set as an elongated rectangle passing through the thinned section 3x along the length direction of the target pipe 3. ROI R4 is set as an elongated rectangle passing through the thinned section 3x along a direction perpendicular to the length direction of the target pipe 3. ROI R3 and R4 are both set based on user input. Figures 6 and 7 correspond to the distribution of pixel values ​​on ROI R3 and R4. The graph in Figure 6 shows the distribution of pixel values ​​displayed on the screen of display 131 by PC120 for ROI R3 in Figure 5, which was set based on user input. The range of pixel values ​​in the graph in Figure 6 is 2121 to 2693. PC120 subdivides ROI R3 into rectangular areas of a predetermined size arranged in a row along the length of ROI R3, and calculates the average value of the pixel values ​​within each rectangular area. Then, PC120 plots the calculated average value on the vertical axis and the position of ROI R3 in the length direction on the horizontal axis, and displays this graph on the screen of display 131 as shown in Figure 6. The graph in Figure 7 shows the distribution of pixel values ​​displayed on the screen of display 131 by PC120 in the same way as in Figure 6 for ROI R4 in Figure 5, based on user input. The range of pixel values ​​in the graph in Figure 7 is 1321 to 2693, which is wider than in Figure 6. The process of displaying the graphs in Figures 6 and 7 corresponds to the graph output process in the present invention. The graphs in Figures 6 and 7 may be output by an output means other than the display 131. For example, they may be recorded on a recording medium by a printing device connected to the PC 120.

[0035] The method for determining the estimated healthy pixel value based on the graphs in Figures 6 and 7 is as follows. First, as shown in Figure 6, an interpolation curve (dotted line) is virtually set, connecting the endpoints of peaks P1 and P2, which represent the pixel values ​​of the thinned area 3x. The interpolation curve is a curve with a shape that smoothly connects the graph between the left and right sides of peaks P1 and P2, assuming that peak P1 did not exist. The interpolation curve is set by drawing a Bézier curve connecting the left and right endpoints of peaks P1 and P2, or by the user drawing it virtually by hand. Next, with respect to the length direction of the ROI, a point on the interpolation curve (intersection with the dashed line) at the same position as the point where the maximum value is taken in peak P1 is selected based on user input from input device 132. The pixel value of this selected point is tentatively determined as the estimated healthy pixel value. Next, similarly for the graph in Figure 7, a point on the interpolation curve is selected based on user input from input device 132. If the user determines that the pixel value of this selected point does not differ significantly from the value tentatively determined based on the graph in Figure 6, the tentatively determined pixel value is formally determined as the estimated healthy pixel value. Alternatively, the average value of the pixel value determined based on the graph in Figure 6 and the pixel value determined based on the graph in Figure 7 may be determined as the estimated healthy pixel value.

[0036] Next, PC120 receives user input of representative pixel values ​​for each part of the reference piece 4 and the thinned portion 3x derived in steps S6 and S8, as well as the estimated healthy pixel value derived in step S9 (step S10). Then, PC120 derives the thickness of the thinned portion 3x and the estimated healthy thickness based on the numerical values ​​input in step S10 (step S11; thickness derivation step in the present invention). The following formula is used to derive the thickness of the thinned portion 3x. (Mathematical formula) logY = P * logX + Q However, X: the thickness of the object in question, Y: the pixel value at the position where you want to find the thickness. The formula is not limited to the one above, and an equivalent formula may be used. An equivalent formula to "logY=P*logX+Q" includes, for example, Y=Q'*(X^P'), which is derived by manipulating the above formula. The "^" indicates exponentiation. The above formula assumes a case where the image is formed such that the pixel value Y increases as the thickness X of the object to be detected decreases. However, the present invention may also be applied to an image formation method in which the pixel value Y' decreases as the thickness X decreases. For example, when applying the present invention to an image formation method in which there is a relationship such as X'=AB*X between X and the object, logY=P*log[(A-X') / B]+Q, obtained by transforming the above formula by X', can be used as an equivalent formula.

[0037] First, P and Q are determined based on the pixel values ​​of each part of the reference piece 4. Specifically, the five pixel values ​​for the reference parts 4A to 4E obtained in step S6 are designated as Y1 to Y5, and the thicknesses of the reference parts 4A to 4E, which were measured in advance, are designated as X1 to X5. Then, P and Q are obtained by performing a curve fitting (fitting) of (X,Y)=(X1,Y1),...(Xn,Yn) to the function logY=P*logX+Q. Any method can be used for curve fitting, such as the least squares method or the maximum likelihood estimation method. Then, the pixel value of the thinned section 3x input in step S10 is set as Y=Ya, or the estimated healthy pixel value is set as Y=Yb, and using the P and Q obtained as described above, as well as the above formula, X=Xa corresponding to Ya and X=Xb corresponding to Y=Yb are obtained, respectively. Both Xa and Xb obtained in this way correspond to the sum of the wall thicknesses of the target pipe 3x through which the transmitted X-rays have passed. The transmitted X-rays pass through two walls of the target pipe 3x: the front wall (the side closer to the X-ray generator 1) and the back wall (the side further from the X-ray generator 1). Therefore, both Xa and Xb are the sum of the thickness of the front wall and the thickness of the back wall. Accordingly, PC120 sets the estimated healthy thickness to half of Xb. Furthermore, PC120 assumes that wall thinning has occurred in either the front or back wall of the target pipe 3, and sets the thickness of the thinned section 3x to the value obtained by subtracting the estimated healthy thickness from Xa.

[0038] Next, the PC120 calculates the thinning rate based on the thickness of the thinned section 3x and the estimated healthy thickness obtained in step S11 (step S12). Specifically, the thinning rate is obtained as a percentage based on (thinning rate) = [(estimated healthy thickness - thickness of thinned section 3x) / (estimated healthy thickness)] * 100. The thickness of the thinned section 3x and the thinning rate obtained in steps S11 and S12 are displayed on the screen of the display 131 (step S13).

[0039] Furthermore, the steps S1 to S3 described above, and the step in step S11 in which P and Q are derived, collectively correspond to the reference derivation process. Also, the steps S1 to S3 described above, and the step in step S11 in which the thickness of the thinned portion 3x is derived, collectively correspond to the main detection process.

[0040] According to the thickness detection method, pipe inspection method, and reference piece of this embodiment described above, the thickness of the target pipe 3 can be easily and quickly detected based on the above formula.

[0041] Furthermore, according to this embodiment, the image of the reference piece 4 is recorded in IP2 along with the target pipe 3. The reference piece 4 is provided with reference sections 4A to 4E, each having a different thickness in the direction of X-ray irradiation, positioned on the reference irradiation surface 5. P and Q are derived based on the pixel values ​​obtained for these reference sections 4A to 4E. By using the above formula with the derived P and Q, the thickness of the target pipe 3 can be easily and quickly detected. Moreover, because the reference piece 4 according to this embodiment has a continuously changing thickness in the direction of X-ray irradiation, it is possible to prevent X-rays from passing through discontinuous steps, thereby suppressing the occurrence of scattering and reducing the edge effect. That is, conventional stepped reference pieces have few flat parts, as shown in the profile (pixel value profile) 500 in Figures 17 and 18, and interference is observed. In contrast, the reference piece 4 according to this embodiment shows continuity, as shown in the profile (pixel value profile) 200 in Figures 9 to 11. This reduces the effort required for evaluators to select areas with low scattering, which is expected to improve work efficiency. In addition, since the density distribution of pixels in the transmission image is obtained continuously, variability in thickness evaluation is suppressed, enabling stable evaluation.

[0042] Furthermore, according to this embodiment, the estimated healthy pixel values ​​are estimated based on the graphs in Figures 6 and 7, assuming that no thinning occurred in the thinned portion 3x of the target pipe 3. The thinned portion 3x appears as a localized peak in the graphs in Figures 6 and 7. Therefore, by assuming a graph without a peak, it is easier to estimate the pixel values ​​of the thinned portion 3x when there is no thinning. Based on the estimated healthy pixel values, the thickness of the thinned portion 3x when there is no thinning (estimated healthy thickness) is estimated, and the thinning rate is derived based on the estimated healthy thickness and the thickness of the thinned portion 3x. In this way, the thickness of the thinned portion 3x can be appropriately evaluated based on the estimated thickness when there is no thinning.

[0043] Furthermore, in this embodiment, when deriving the estimated sound thickness, an interpolation curve is set in the graphs of Figures 6 and 7 that shows the shape of the position corresponding to the thinned portion 3x, assuming that no thinning occurred. Then, the estimated thickness is derived based on the pixel value for a point on the interpolation curve and the above formula. Therefore, the estimated thickness can be appropriately derived based on the interpolation curve assuming no thinning occurred. Moreover, in this embodiment, the estimated sound thickness is derived based on the two graphs shown in Figures 6 and 7. Therefore, the estimated sound thickness can be accurately derived.

[0044] Although preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications are possible without departing from the spirit of the invention. Hereinafter, modifications of the above embodiments will be described.

[0045] The logarithmic relationship between thickness and pixel value is approximated as a curve corresponding to the position from the edge of the reference illumination surface 5. Therefore, in order to adjust for the effects of unwanted scattering and diffraction caused by the linear wedge shape and to improve the accuracy of the approximation to this curve relationship, the shape of the reference illumination surface 5 may be changed as follows. For example, the reference illumination surface 5 may be a surface that curves outward or inward of the reference piece 4 as shown in Figures 12 and 13 (reference numerals 410, 420), or it may be a surface whose width (height) changes from one end to the other as shown in Figure 14 (reference numeral 430).

[0046] Furthermore, in the above-described embodiment, the object whose thickness was to be detected was the target pipe 3, but this embodiment may also be applied to objects other than pipes.

[0047] Furthermore, in the above embodiment, the target pipe 3 and the reference piece 4 are irradiated with X-rays simultaneously, and their images are recorded in IP2. However, the images of the target pipe 3 and the reference piece 4 are not recorded simultaneously in IP2, but may be recorded at different timings. For example, P and Q may be calculated in advance based on the image of the reference piece 4 before acquiring the image of the target pipe 3.

[0048] Furthermore, in the above-described embodiment, five reference sections 4A to 4E are provided so as to be located on the reference irradiation surface 5. However, the number of reference sections may be two to four or six or more.

[0049] Furthermore, in the above-described embodiment, it was assumed that an image analysis application and a thickness derivation application were installed on PC120. However, a single application that combines the functions of both of these applications may be installed on PC120. Alternatively, three or more applications that share the functions of these two applications may be installed on PC120. The thickness derivation application can be any type of application that performs predetermined calculations based on user-inputted values. For example, it may be a spreadsheet program. In addition, some functions of the image analysis application and the thickness derivation application may be shared by one or more computers other than PC120.

[0050] Furthermore, in the above-described embodiment, Figure 6 or Figure 7 is a graph displayed on the display 131 with respect to ROI R3 or R4 set along the longitudinal direction of the target pipe 3. However, instead of the graph in Figure 6 or Figure 7, a graph relating to an ROI set slightly obliquely to the longitudinal direction of the target pipe 3 or a direction perpendicular thereto may be used. For example, a graph relating to an ROI along a direction in which the angle with respect to the longitudinal direction of the target pipe 3 is less than 45° may be used instead of the graph in Figure 6. Also, a graph relating to an ROI along a direction in which the angle with respect to the longitudinal direction of the target pipe 3 and a direction perpendicular thereto is less than 45° may be used instead of the graph in Figure 7.

[0051] Furthermore, in the above-described embodiment, ROIs R3 and R4 are set as regions having a certain width. Figures 6 and 7 are plotted as graphs showing the average value for each rectangular region of a predetermined size arranged in a row along the length direction of each ROI. However, ROIs R3 and R4 may be set as regions without width, that is, as linear regions. In this case, instead of the graphs in Figures 6 and 7, graphs showing the pixel values ​​of each pixel that overlaps the linear ROIs R3 and R4 may be plotted.

[0052] Furthermore, in the above-described embodiment, the thickness of the thinned portion 3x is derived based on the assumption that the wall thinning of the target pipe 3 occurs on either the front or rear side of the wall. Whether the wall thinning of the target pipe 3 occurs on either the front or rear side or on both sides needs to be determined, for example, by observing an X-ray transmission image at a different angle than the X-ray transmission image in the above-described embodiment. If it is determined that the wall thinning of the target pipe 3 occurs on both the front and rear sides, it is preferable that the rate of thinning of the thinned portion 3x be evaluated as being smaller than the value obtained when it is assumed that the wall thinning of the target pipe 3 occurs on either the front or rear side.

[0053] Furthermore, the thickness of the thinned portion 3x of the target pipe 3 derived by the above-described embodiment is, strictly speaking, the thickness with respect to the transmission direction of the X-rays passing through the target pipe 3. For example, in the case of portion 3a shown in Figure 8, the radial direction of the target pipe 3 and the transmission direction of the X-rays coincide. Therefore, the derived thickness of portion 3a is the thickness with respect to the radial direction. However, in the case of portion 3b shown in Figure 8, the radial direction of the target pipe 3 and the transmission direction of the X-rays do not coincide. In this case, the thickness derived for portion 3a will be the thickness along the X-ray transmission direction which is different from the radial direction. Therefore, the thickness derived by the above-described embodiment may be corrected to the thickness along the radial direction of the target pipe 3. Specifically, the thickness obtained by multiplying the derived thickness by the cosine of the angle between the radial direction and the transmission direction of the X-rays may be taken as the thickness with respect to the radial direction. Note that in order to perform such a correction, it is necessary to estimate whether the thinning of the wall portion of the target pipe 3 is occurring on the front side or the back side. This estimation may be done, for example, by observing an X-ray transmission image at a different angle than the X-ray transmission image in the above-described embodiment. Furthermore, the direction and distance from the X-ray light source differ depending on the detection position. For example, as shown in Figure 8, the distance and direction from the X-ray generator 1 in section 3a are different from those in section 3b. Therefore, the dose of X-rays irradiated onto the target pipe 3 differs depending on the detection position. In the above embodiment, the difference in irradiation dose due to the difference in detection position is not taken into consideration, based on the assumption that such a difference in irradiation dose is not very large. However, P and Q may be calculated, and the thickness of the thinned section 3x may be calculated, taking into account the difference in X-ray irradiation dose due to the difference in position on the target pipe 3 and the reference piece 4.

[0054] The terms and expressions used in this specification are for illustrative purposes only, not limiting purposes, and the use of such terms and expressions does not exclude any equivalents of the illustrated and described features or parts thereof from the scope of the present invention. [Explanation of Symbols]

[0055] 1. X-ray generator 2 IP (Imaging Plates) 3. Target piping 3x Thinner area 4 Reference piece 4A~4E Reference part 5 Reference irradiation surface 10 X-ray generator 20 IP (imaging plates) 30 Target piping 40 Reference piece 40A~40E Reference section 100 Image Analysis Systems 110 IP Reader 120 PCs (Computers) 200 Profiles (Pixel Value Profiles) 400 reference piece 401 Reference piece 402 Reference piece 500 Profiles (Pixel Value Profiles)

Claims

1. An irradiation step in which X-rays that have passed through the object to be detected are irradiated onto a photostimulable phosphor held on a plate, The image generation step involves reading the image of the object to be detected recorded on the photostimulable phosphor of the plate during the irradiation step, and generating an X-ray transmission image showing the object to be detected. The system includes a thickness derivation step, in which the thickness of the object to be detected is denoted as X, and the pixel value of the pixel in the X-ray transmission image generated in the image generation step is denoted as Y, and the thickness of the object to be detected is denoted as X, and the thickness of the object to be detected is denoted as Y, based on a predetermined relationship that X and Y satisfy. The aforementioned predetermined relationship is such that when P and Q are constants, logY=P*logX+Q Alternatively, a thickness detection method expressed by an equivalent mathematical formula, Instead of the object to be detected, the following steps are performed with respect to a reference piece having a reference irradiation surface in which the thickness in the direction of X-ray irradiation continuously changes and which curves outward or inward, and the first to n reference parts having thicknesses X1 to Xn (n: an integer of 2 or more) respectively, which are located on the reference irradiation surface: the irradiation step and the image generation step are performed, and a reference derivation step is performed to derive P and Q such that when Yi (i: an integer of 1 or more and n or less) is the pixel value relating to the i reference part, (X, Y) = (Xi, Yi) satisfies or approximately satisfies the predetermined relationship for i = 1, 2, ... n, A thickness detection method comprising: performing the two steps described above on the object to be detected; and a main detection step in which the thickness of the object to be detected is detected based on the predetermined relationship using P and Q derived in the reference derivation step.

2. The reference portion is defined as a position that is a certain distance or more away from the edge of the aforementioned reference irradiation surface. The thickness detection method according to claim 1.

3. A pipe inspection method characterized by detecting the thickness of a pipe as the object to be detected using the thickness detection method described in claim 1 or 2.

4. A graph output step involves outputting a graph to an output means that shows the distribution of the pixel values ​​on a virtual line passing through the thinned portion of the pipe or its vicinity along the X-ray transmission image, The system includes a sound thickness estimation step, which derives the estimated thickness of the thinned portion assuming that no thinning occurred, based on the graph output by the output means. In the detection step described above, the thickness of the thinned portion is evaluated relative to the estimated thickness. The pipe inspection method according to claim 3.

5. The estimated thickness is derived based on the pixel value of a point on the interpolation curve that shows the shape of the graph at the position corresponding to the thinned portion, assuming that no thinning occurred, and the predetermined relationship. The pipe inspection method according to claim 4.

6. In the graph output step, the graph is output to the output means for each of the two different virtual lines. In the healthy thickness estimation step, the thickness of the thinned portion is estimated based on the two graphs relating to the two imaginary lines, assuming that no thinning occurred. The pipe inspection method according to claim 5.

7. X-rays transmitted through the object to be detected are irradiated onto a photostimulable phosphor held on a plate, the image of the object to be detected recorded on the photostimulable phosphor is read, and an X-ray transmission image showing the object to be detected is generated. When the thickness of the object to be detected is X and the pixel value of the pixels in the X-ray transmission image is Y, then X and Y are constants, and P and Q are constants. logY=P*logX+Q Alternatively, a reference piece used in the derivation of P and Q that satisfies or approximately satisfies a predetermined relationship expressed by an equivalent mathematical formula, The X-ray irradiation direction has a reference irradiation surface in which the thickness of the material changes continuously and which curves outward or inward. A reference piece in which first to n reference portions, each having a thickness of X1 to Xn (n: an integer of 2 or more), are located on the reference irradiation surface.

8. The reference portion is spaced a certain distance or more from the edge of the reference irradiation surface. The reference piece according to claim 7.

9. A reference piece used to derive a mathematical formula that expresses a predetermined relationship that X and Y satisfy, where X is the thickness of the object to be detected, and Y is the pixel value of the pixel in the X-ray transmission image generated by reading the image of the object to be detected recorded on a photostimulable phosphor that has been irradiated with X-rays that have passed through the object to be detected, The X-ray irradiation direction has a reference irradiation surface in which the thickness of the material changes continuously and which curves outward or inward. A reference piece in which first to n reference portions, each having a thickness of X1 to Xn (n: an integer of 2 or more), are located on the reference irradiation surface.

10. The reference portion is spaced a certain distance or more from the edge of the reference irradiation surface. The reference piece according to claim 9.

Images