JP2009276285A - Apparatus and method for radiation tomographic photographing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reconstruct tomographic images with high image quality from radiograph data by achieving both a high amount of radiation and a high-gain setting of a measuring circuit. <P>SOLUTION: A radiographic apparatus 11 is provided with a shielding body 74 for shielding radiation irradiated from a radiation source in some of a plurality of channels of a radiation detector 2. A storage device 31 of a control computational unit 12 stores a channel constant k(u, v) composed of a ratio (I0 (u, v)/I_ref) of output I0 (u, v) of each channel except a reference channel of the radiation detector 2 and output I_ref of the reference channel with an analyte 74 not mounted between the radiation source 1 and the radiation detector 2. An image reconstruction device 22 computes a decay rate a(u, v) of each channel (u, v) by multiplying a channel constant k(u, v) by an output value I_ref of the reference channel, dividing its product by output I(u, v) of each channel with the analyte mounted, and logarithmically converting its quotient. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a radiation tomography apparatus and a radiation tomography method, and more particularly, to a radiation nondestructive inspection system and piping suitable for visualizing and inspecting the inside of a structure installed at a specific place such as piping. It relates to the inspection method.

  Radiation such as X-rays and γ-rays is applied to non-destructive inspection methods that visualize and inspect the inside of structures installed at specific locations, such as piping installed in nuclear power plants, thermal power plants, chemical plants, etc. There is a radiographic test (RT) used. RT measures the radiation irradiated to the structure (subject) to be examined with a radiation detector placed on the opposite side of the radiation source across the subject, and takes a two-dimensional transmission image of the subject It is a method to do. In the examination by RT, confirmation of the state inside the subject and measurement of the dimensions are performed using the photographed transmission image.

  In an apparatus that checks the thinning of plant piping by RT, a radiation source and a high-sensitivity image intensifier (radiation detector) are attached to a support mechanism called a C-shaped arm, the plant piping is scanned, and the transmission image of the piping Is known, and the amount of thinning is measured from the transmission image (see, for example, Patent Document 1).

  As another nondestructive inspection method using radiation, computed tomography (CT) is known. An industrial X-ray CT apparatus generally fixes an X-ray source and a radiation detector, and rotates a subject placed on a disk disposed between them. Then, by irradiating the subject with radiation while rotating the subject, a transmission image is taken from all around the subject, and a tomographic image of the subject is obtained by image reconstruction. The difference from the transmission image by RT is that a three-dimensional image inside the subject is obtained. For this reason, more detailed position information can be acquired about the internal structure of the subject.

  Furthermore, as a radiation CT apparatus for nondestructive inspection of piping, an imaging apparatus based on an imaging method called laminography for inspecting a joint of a pipe weld is known (for example, see Patent Document 2). In laminography, a radiation source and a radiation detector installed so as to face each other with a subject sandwiched between each other are moved in parallel and in opposite directions, thereby obtaining a tomographic image parallel to the movement direction of the radiation source and the radiation detector. It is a method of shooting. By changing the moving distance of the radiation detector with respect to the moving distance of the radiation source, the depth of the imaging tomographic plane viewed from the radiation source can be changed.

Toshiba Review, Vol.61, No.6 (2006), Tomita, Katayama, “Piping thickness inspection device”, pp.68-71 16th WCNDT proceedings, (2004), B. Redmer, et. Al, "MOBLE 3D-X-RAY TOMOGRAPHY FOR ANALYSIS OF PLANAR DEFECTS IN WELDS BY TOMOCARR"

  However, the RT has a problem that the appearance of the thinned portion generated in the pipe to be inspected changes depending on the photographing direction. For this reason, in RT, it is necessary to image the subject from a plurality of directions in order to search for an imaging direction in which the thinned portion of the pipe can be observed. This operation usually takes several minutes to several tens of minutes. It was.

  CT also requires that the subject be rotated or that the radiation source and radiation detector be rotated around the subject. However, in the case of piping installed in the plant, it is impossible to rotate the piping. Further, the circumference of the pipe is narrow, and there is no room for rotating the radiation source and the radiation detector around the pipe, so that CT imaging is difficult.

  Furthermore, in tomography of pipes by laminography, the tomography is performed with the radiation detector fixed, so that one imaging range depends on the detector size of the radiation detector. Therefore, it takes time to shoot the entire long pipe.

  In order to avoid these problems, recently, a radiation source and a radiation detector installed so as to face each other with the subject interposed therebetween are moved in parallel with each other in the same direction. An apparatus for taking a transmission image parallel to the direction of motion and reconstructing a tomographic image using necessary portions from the transmission image data has been studied.

  In any of the methods described above, a sealed gamma ray source or an X-ray tube is usually used as the radiation source. Usually, a two-dimensional radiation detector is used as the radiation detector. There are two-dimensional detectors that convert radiation into light with a scintillator film, receive the light with a CCD or TFT panel and convert it into charges, or detect radiation directly with a semiconductor detector and convert it into charges. It is called FPD (Flat Panel Detector). Because these FPDs have high shape resolution, the individual detectors (pixels) are small, so that the amount of charge due to radiation that can be stored in one data measurement (ie, data reading from the FPD) is limited, and many radiations Is incident, the output is saturated and cannot be used as data.

  In radiation measurement, the fluctuation of the output signal is proportional to the square of the number of radiation photons detected by the detector, so the S / N (signal to noise ratio) of the detector output increases as the number of detected photons increases. For this reason, it is desirable to use a radiation source with a dose rate as high as possible, but there is a possibility that the detector is saturated as described above.

  Since radiation attenuates when it passes through the subject, it is necessary to increase the intensity of the radiation source in order to image the subject portion with high image quality. However, for this reason, the radiation output is too large and the detector output is saturated in a portion where there is no subject (hereinafter referred to as an air layer) or a thin portion of the subject for the above reason. In the two-dimensional detector, since the outputs of a large number of detectors (pixels) are read in series, the gain of the measurement circuit cannot be changed for each pixel. For this reason, since the gain of the measurement circuit must be set low, there is a problem that the output of the subject portion becomes small and the error in converting to a digital value becomes large.

  In order to reconstruct a tomographic image, it is necessary to use measurement data of an air layer or a thin part of a subject, and it is necessary to weaken the radiation dose. When weakened, the detector output of the subject portion becomes small, photon noise increases, and a contradiction arises that S / N deteriorates.

An object of the present invention is to provide a radiation tomography apparatus capable of reconstructing a tomographic image with high image quality from radiation transmission image data by satisfying both a high radiation dose necessary for high S / N of measurement data and a high gain setting of a measurement circuit. It is to provide a radiation tomography method.

(1) In order to achieve the above object, the present invention provides a radiation imaging apparatus having a radiation source and a two-dimensional radiation detector, which are opposed to each other with a subject interposed therebetween. A radiation tomography apparatus comprising: a control arithmetic unit that constructs a tomographic image of the subject from a collection of transmission data of the subject obtained by detecting radiation transmitted through the sample by the radiation detector, The radiation imaging apparatus includes a shielding body that shields radiation emitted from the radiation source in a part of the plurality of channels of the radiation detector, and a channel shielded by the shielding body as a reference channel. The control arithmetic unit is configured to perform the reference of the radiation detector in a state where the subject is not installed between the radiation source and the radiation detector. Channel constant k (u, v) comprising the ratio (I0 (u, v) / I_ref) of the output I0 (u, v) of each channel other than the channel to the output I_ref of the reference channel of the radiation detector And a reference in a state in which the subject is installed between the radiation source and the radiation detector by multiplying the channel constant k (u, v) by the output value I_ref of the reference channel. An image reconstruction device that calculates an attenuation factor a (u, v) of each channel (u, v) as a logarithmically converted one obtained by dividing the output I (u, v) in each channel other than the channel It is intended to provide.
With such a configuration, a high radiation dose necessary for increasing the S / N ratio of measurement data and a high gain setting of the measurement circuit are compatible, and a tomographic image can be reconstructed from radiation transmission image data with high image quality.

(2) Further, in order to achieve the above object, the present invention provides a radiation source and a radiation detector facing each other across the subject, and from the collected transmission data of the subject, A radiation tomography method for constructing a tomographic image, wherein the subject is not installed between the radiation source and the radiation detector, and the radiation detector includes a channel from the radiation source in each channel of the radiation detector. Ratio (I0 (u, v) / I_ref) of the output I_ref of the reference channel where the irradiated radiation is shielded by the shield and the output I0 (u, v) of the channel other than the reference channel of the radiation detector The channel constant k (u, v) is calculated, and the channel constant k (u, v) is multiplied by the output value I_ref of the reference channel to obtain the radiation. Obtained by dividing by the output I (u, v) in each channel other than the reference channel in a state where the subject is installed between the radiation detector and the radiation detector, and each channel (u, v) The attenuation rate a (u, v) of the above is calculated.
With this method, it is possible to reconstruct a tomographic image with high image quality from radiation transmission image data while achieving both a high radiation dose required for high S / N of measurement data and a high gain setting of the measurement circuit.

(3) Furthermore, in order to achieve the above object, the present invention is directed to a radiation imaging apparatus having a radiation source and a two-dimensional radiation detector that are arranged to face each other across a subject, and irradiated from the radiation source. A radiation tomography apparatus comprising: a control arithmetic unit that constructs a tomographic image of the subject from a collection of transmission data of the subject obtained by detecting radiation transmitted through the subject by the radiation detector. The radiation imaging apparatus includes a monitor detector that is arranged at a position where radiation irradiated from the radiation source is always irradiated and has a dynamic range wider than each channel of the radiation detector, , Output I0 (u, v) of each channel of the radiation detector in a state where the subject is not installed between the radiation source and the radiation detector; A storage device for storing a channel constant k (u, v) having a ratio (I0 (u, v) / I_ref) to the output I_ref of the monitor detector, and the monitor for the channel constant k (u, v) Logarithmically converted product obtained by multiplying the output value I_ref of the detector and dividing by the output I (u, v) in each channel when the subject is installed between the radiation source and the radiation detector And an image reconstruction device that calculates the attenuation rate a (u, v) of each channel (u, v).
With this configuration, a high radiation dose necessary for increasing the S / N ratio of measurement data and a high gain setting of the measurement circuit are compatible, and a tomographic image can be reconstructed from radiation transmission image data with high image quality.

(4) In order to achieve the above object, the present invention provides a tomographic image of the subject from a set of transmission data of the subject obtained by arranging a radiation source and a radiation detector facing each other across the subject. A tomography method for constructing a radiological tomography, comprising: an output I0 (u, v) of each channel of the radiation detector when the subject is not installed between the radiation source and the radiation detector; The ratio (I0 (u, v) /) of the output I_ref of the monitor detector which is arranged at a position where the radiation from the radiation source is always irradiated and has a wider dynamic range than each channel of the radiation detector. A channel constant k (u, v) consisting of I_ref) is calculated, the channel constant k (u, v) is multiplied by the output value I_ref of the monitor detector, and the radiation source and the radiation are calculated. An attenuation factor a (u) of each channel (u, v) is obtained by logarithmically transforming the output I (u, v) in each channel when the subject is placed between the detector and the detector. , V) is calculated.
With this method, it is possible to reconstruct a tomographic image with high image quality from radiation transmission image data while achieving both a high radiation dose required for high S / N of measurement data and a high gain setting of the measurement circuit.

  According to the present invention, a high radiation dose necessary for increasing the S / N ratio of measurement data and a high gain setting of the measurement circuit are compatible, and a tomographic image can be reconstructed with high image quality from radiation transmission image data.

Hereinafter, the configuration and operation of a radiation tomography apparatus according to an embodiment of the present invention will be described with reference to FIGS.
In the radiation tomography apparatus according to this embodiment, the radiation source and the radiation detector are moved in the same direction, a transmission image parallel to the movement direction is taken, and a tomogram is obtained from the transmission image data using a necessary portion. Reconfigure. In the present embodiment, a case where such a radiation tomography apparatus performs nondestructive inspection using a pipe equipped with a heat insulating material installed in a power plant or the like as a subject is described.

First, the system configuration of the radiation tomography apparatus according to the present embodiment will be described with reference to FIG.
FIG. 1 is a block diagram showing a system configuration of a radiation tomography apparatus according to an embodiment of the present invention.

  The radiation tomography apparatus according to the present embodiment includes a radiation imaging apparatus 11 and a control arithmetic device 12.

  The radiation imaging apparatus 11 includes a radiation source 1, a radiation detector 2, and a scanner device 3 that supports them. The radiation source 1 is an X-ray tube. As the radiation detector 2, a two-dimensional radiation detector is used in which a CCD (Charge Coupled Device) panel or a TFT (Thin Film Transistor) panel is installed behind the scintillator surface that emits light when radiation is incident. A shield 74 that can shield the radiation emitted from the radiation source 1 is provided on a part of the upper surface of the detector panel 71 of the radiation detector 2. The scanner device 3 has a function of performing translational scanning in the major axis direction of the pipe 10 on which the heat insulating material 99 is mounted and in the same direction while maintaining the positional relationship between the radiation source 1 and the radiation detector 2.

  The radiation source 1 can control the output current from 0 to several milliamperes by a command from the control device 21. The amount of X-ray generation can be controlled by changing the output current. The radiation detector 2 includes a detector panel 71 composed of a number of detectors (hereinafter referred to as “channels”), an amplifier circuit 72 that amplifies the output of each pixel output from the detector panel 71, and an analog of the amplifier. And an AD conversion circuit 73 for converting the output into a digital value. The output of the AD conversion circuit 73 is sequentially transferred to the control device 21 as the transmission image data 51. The gain of the amplifying circuit 72 can be changed by a command from the control device 21, but it cannot be changed for each pixel, and is set to an appropriate gain value before measurement.

  The control arithmetic device 12 includes an image capturing device 20, a control device 21, an image reconstruction device 22, a determination device 24, a storage device 31, and a reconstructed image storage device 32.

  The image capturing device 20 captures a plurality of transmission image data 51 photographed by the radiation detector 2. The control device 21 controls the scanner device 3, the radiation source 1, and the radiation detector 2. The storage device 31 stores a plurality of transmission image data 51 captured by the image capturing device 20 and a channel constant 75 of each pixel of the radiation detector 2. The present embodiment is characterized in that the shield 74 provided on the upper surface of the detector panel 71 of the radiation detector 2 and the channel constant 75 are used, which will be described later.

  The determination device 24 reads a plurality of transmission image data 51 from the storage device 31 and displays them as a still image or a moving image on a monitor such as a PC. Based on the displayed image, the operator needs to reconstruct the image. This is used to input the determination result and the image reconstruction area. The image reconstruction device 22 reconstructs a tomographic image or a three-dimensional stereoscopic image of the subject. The reconstructed image storage device 32 stores the tomographic image or three-dimensional stereoscopic image (reconstructed image) of the subject reconstructed by the image reconstructing device 22.

  In consideration of user convenience, an image measurement device 23 equipped with image measurement software or the like for performing image measurement using a tomographic image or a three-dimensional stereoscopic image (reconstructed image) can be added as necessary. It is.

Next, the configuration of the radiation detector 2 used in the radiation tomography apparatus according to the present embodiment will be described with reference to FIG.
FIG. 2 is a top view showing a configuration of a radiation detector used in the radiation tomography apparatus according to the embodiment of the present invention. The same reference numerals as those in FIG. 1 denote the same parts.

  A shield 74 is provided on a part of the upper surface of the detector panel 71 of the radiation detector 2. The installation position of the shield 74 is always a position where an X-ray beam that has passed through only the air layer is directly incident, not an X-ray transmitted through the subject. That is, the X-ray irradiated from the radiation source 1 is provided at a position where it is not blocked by the subject. The thickness of the shield 74 depends on the material and thickness of the subject, the sensitivity and output of the radiation detector, the distance between the X-ray tube 1 and the radiation detector 2, the output of the X-ray tube, and the amplification circuit 72. It is necessary to determine appropriately according to the setting gain. This will be described later.

Next, the inspection method in the radiation tomography apparatus according to the present embodiment will be described with reference to FIG.
FIG. 3 is a flowchart showing an inspection method in the radiation tomography apparatus according to the embodiment of the present invention.

  The inspection method of the present embodiment includes an initial setting flow 3001 before subject imaging, an imaging flow 2001 for acquiring transmission image data by imaging, determination of necessity of image reconstruction based on the acquired transmission image data, and image reconstruction. And an image processing flow 2002 for implementing the configuration.

  In the initial setting flow 3001, the system captures a transmission image while changing the X-ray tube current from 0 to the maximum value in a state where the system is not installed on the subject (process 3003). The positional relationship between the X-ray tube 1 and the radiation detector 2 in this state is set to be the same as that in the subsequent subject imaging. During this processing, the X-ray tube 1 and the radiation detector 2 do not need to move. Using the transmission image data, a channel constant 75 is obtained from the output relationship of the channel with the shield and the output relationship of the other channels (processing 3004) and stored in the storage device 31 (processing 3005).

  The channel constant 75 is calculated from the output I_ref of the reference channel (u1, v1) with the shield and the air layer output I0 (u, v) of the other channel (I0 (u, v) / I_ref). Is a value calculated as

  In the imaging flow 2001, the scanner apparatus 3 moves the radiation source 1 and the radiation detector 2 in the longitudinal direction of the pipe, and starts translational scanning (processing 2003). The radiation source 1 emits radiation at the current scanner position, and captures a transmission image when the radiation that has passed through the piping enters the radiation detector 2 (processing 2004). The transmission image data captured by the image capturing device 20 from the radiation detector 2 is stored in the storage device 31 (process 2005).

  Then, the control device 21 moves the scanner device 3 by a specified distance in the long axis direction of the pipe (process 2006), and determines whether the scanner apparatus 3 has reached the end of the pipe (process 2007). In process 2007, when the scanner device 3 has not reached the end of the pipe, the process returns to process 2004, and a transmission image at the current scanner position is captured. On the other hand, when the scanner device 3 reaches the end of the pipe, the photographing is finished (processing 2008).

  In the image processing flow 2002, the transmission image data stored in the storage device 31 is read and displayed on the screen of the determination device 24 as necessary (processing 2009). The plurality of transmission image data 51 are displayed as a still image or a moving image on the screen.

  Based on the read transmission image data, the operator visually confirms the plurality of displayed transmission image data 51 and determines whether or not it is necessary to reconstruct a three-dimensional image of the pipe in order to observe the three-dimensional shape of the pipe. (Processing 2010 and Processing 2011). The determination as to whether reconfiguration is necessary may be executed based on an operator's judgment criteria, or some criteria may be determined in advance using a standard document, and may be executed based on the criteria. An image reconstruction area is designated, input to the determination device 24, and a reconstruction process is instructed (process 2013).

  In response to the execution instruction from the determination device 24, the image reconstruction device 22 performs image reconstruction processing using the transmission image data 51 and the channel constant data 75 of the corresponding part, and outputs a three-dimensional tomographic image of the corresponding part (2013). ). If the read transmission image data is at the end of the pipe (inspection object), the process ends. If not, the reading process 2010 is repeated (process 2014). If it is determined in step 2011 that image reconstruction is unnecessary, it is determined whether the transmitted image data is data at the end of the pipe (step 2014).

  In the above description, shooting and image processing are performed in parallel, but image processing may be performed after all shooting is completed. Moreover, the measured raw data is used as the transmission image data, and the data of the air layer portion without the subject is saturated. Instead, as described later, the attenuation rate of each channel can be calculated using the transmission image data 51 and the channel constant data 75, and the attenuation rate data 55 corresponding to the transmission image can be displayed as an image.

Next, a method for generating a tomographic image or a three-dimensional stereoscopic image (reconstructed image) of the subject in the radiation tomography apparatus according to the present embodiment will be described with reference to FIGS.
First, a specific configuration of the radiation tomography apparatus according to the present embodiment will be described with reference to FIG.
FIG. 4 is a perspective view showing a specific configuration of the radiation tomography apparatus according to the embodiment of the present invention. The same reference numerals as those in FIG. 1 denote the same parts.

  In the radiation imaging apparatus 11, the radiation source 1 and the radiation detector 2 are held by a C-shaped arm 3a. The C-shaped arm 3a is scanned on the guide rail 3b. The guide rail 3b is arrange | positioned along the major axis direction of the piping 10 with which the heat insulating material (not shown) was mounted | worn by the support leg 3c installed on the floor surface. Further, the C-shaped arm 3 a is formed along the outer peripheral surface of the pipe 10, and the radiation source 1 and the radiation detector 2 are arranged to face each other with the pipe 10 interposed therebetween.

  Each time the C-shaped arm 3a moves a certain distance in the long axis direction of the pipe, one piece of transmission image data is measured. Since an X-ray tube is used as the radiation source 1, X-rays are continuously irradiated. Transmission image data 51 photographed by the radiation detector 2 while the C-shaped arm 3a is scanning the pipe is captured from the radiation detector 2 to the image capturing device 20 as needed. A general two-dimensional radiation detector outputs transmission image data of 30 sheets / second.

  The control arithmetic unit 12 has a configuration as shown in the figure. Each part of the control arithmetic unit 12 is as described in FIG.

Next, the imaging state of the radiation tomography apparatus according to the present embodiment will be described with reference to FIG.
FIG. 5 is an explanatory diagram of an imaging state in the radiation tomography apparatus according to the embodiment of the present invention. The same reference numerals as those in FIG. 1 denote the same parts.

  As shown in FIG. 5, the radiation source 1 and the radiation detector 2 move while being held by the C-shaped arm, and are transmitted at a predetermined timing, that is, whenever they move by a certain distance in the long axis direction of the pipe. Image data 51 is measured. Thereby, a plurality of pieces of transmission image data are obtained along the pipe.

Next, the collection range of transmission image data necessary for image reconstruction in the radiation tomography apparatus according to the present embodiment will be described with reference to FIG.
FIG. 6 is an explanatory diagram of a transmission image data collection range necessary for image reconstruction in the radiation tomography apparatus according to the embodiment of the present invention. The same reference numerals as those in FIG. 1 denote the same parts. For ease of explanation, FIG. 6 shows the case of two-dimensional imaging. In the case of three-dimensional imaging, this two-dimensional concept is expanded.

  Consider the case where the radiation source 1 and the radiation detector 2 perform translational scanning from left to right as shown in FIGS. Further, a plate-like object 10a is considered as a subject, and attention is paid to a point 10b inside the object 10a.

  The radiation 5 transmitted through the internal point 10b starts translational scanning in the direction shown in FIG. 6A, passes through in the direction shown in FIG. 6B, and ends in the direction shown in FIG. 6C. . If the opening angle of the radiation is θ, the angle range of the radiation 5 that passes through the internal point 10b during this translational scanning is also θ. Generally, in order to reconstruct a tomographic image by CT imaging, it is necessary to transmit radiation from a direction of 180 ° to 360 ° with respect to the subject.

  In contrast, in the radiation imaging apparatus 11 of the present embodiment, the radiation transmission direction is an angle θ. This angle θ is determined by the radiation angle of the radiation source 1 or the size of the detection surface of the radiation detector 2, and is about 40 ° to 60 °. In order to perform image reconstruction under such a condition, a technique (Limited Angle image reconstruction) is required in which image reconstruction is performed in a state where the projection angle is limited.

  Many limited angle image reconstruction methods have been proposed. In the following, an image reconstruction method will be described using the DTS (Digital Tomosynthesis) method, which is one of the methods, as an example. Of course, other limited angle image reconstruction techniques can be applied.

Next, the principle of image reconstruction by the DTS method in the radiation tomography apparatus according to the present embodiment will be described with reference to FIG.
FIG. 7 is an explanatory diagram of the principle of image reconstruction by the DTS method in the radiation tomography apparatus according to the embodiment of the present invention. The same reference numerals as those in FIG. 1 denote the same parts.

  Here, for simplicity of explanation, it is assumed that only the radiation source 1 moves in translation and the radiation detector 2 is fixed.

  The subjects are a circular subject 10c and a rectangular subject 10d having no thickness. It is assumed that the subject 10c and the subject 10d are arranged in parallel to the radiation detector 2 and perpendicular to the direction axis from the radiation source 1 toward the radiation detector 2. Further, it is assumed that the subject 10c and the subject 10d have different distances from the radiation source 1.

  For the subjects 10c and 10d, transmission image data 51 as shown in the figure is photographed corresponding to each position when the radiation source 1 performs translational scanning. When reconstructing a cross section including the circular subject 10c from these transmission image data 51, after moving each transmission image data 51 so that the projection part of the subject 10c in each transmission image data 51 overlaps, The transparent image data 51 is superimposed. By this processing, a circular image becomes clear. The amount of movement of each transmission image data 51 includes the amount of movement of the radiation source 1 during imaging of each transmission image data 51, the distance between the radiation source 1 and the cross section including the circular subject 10c, and the circular subject 10c. It is determined by the distance between the included cross section and the radiation detector 2.

  On the other hand, the projection part of the subject 10d in each transmission image data 51 becomes an unclear image due to the movement and overlay processing of the transmission image data 51. As a result, a contrast difference occurs between the circular subject 10c and the rectangular subject 10d, and a tomographic image 52 that is a reconstructed image of the subject 10c can be generated.

  Similarly, a subject 10d located at a depth position different from that of the subject 10c with respect to the radiation source 1 can generate a reconstructed image by the same method as the above-described reconstruction method. As described above, since a tomographic image or a three-dimensional image of a pipe can be reconstructed using a transmission image of the pipe taken for screening the pipe, it is not necessary to re-acquire data.

Next, a case where a limited angle reconstruction image technique such as the DTS method in the radiation tomography apparatus according to the present embodiment is applied to pipe imaging will be described with reference to FIG.
FIG. 8 is an explanatory diagram when a limited angle reconstructed image technique such as the DTS method is applied to piping imaging in the radiation tomography apparatus according to the embodiment of the present invention. The same reference numerals as those in FIG. 1 denote the same parts.

  When a limited angle reconstruction image technique such as the radiation imaging apparatus 11 and the DTS method is applied to pipe imaging, the reconstruction image 52 has an axis from the radiation source 1 toward the radiation detector 2 as shown in FIG. Generated as a cross section with parallel normal vectors. Further, a plurality of reconstructed images 52 having different distances from the radiation source 1 are generated between the radiation source 1 and the radiation detector 2. A three-dimensional stereoscopic image 53 can be constructed by stacking the reconstructed images 52 in the axial direction from the radiation source 1 toward the radiation detector 2.

  Next, the processing flow of image reconstruction will be described.

First, conventional image reconstruction processing will be described with reference to FIGS.
FIG. 17 is a flowchart showing the processing contents of conventional image reconstruction. FIG. 18 is an explanatory diagram of data used in the conventional image reconstruction method. FIG. 19 is an explanatory diagram of the relationship between the X-ray intensity and the detector output in the conventional image reconstruction method.

  First, the processing content of the conventional image reconstruction will be described with reference to FIG.

  First, a condition input process 9001 for inputting calculation conditions such as input data names and calculation parameters is executed. Next, based on the input calculation conditions, the image reconstruction device 9002 transmits the transmission image data captured by the radiation imaging device from the storage device, and the air layer data reads the air layer data from the storage device. A read process 9003 is executed. In the transmission image data reading process 9002 and the air layer data reading process 9003, the first transmission image data obtained by photographing the part of the pipe that needs to be reconstructed in order to confirm the state of the defective part of the pipe, The second transparent image data captured before and after the first transparent image data is read. Air layer data refers to data taken in the absence of a subject, and is obtained with radiation intensity without attenuation. This data is used in the next processing.

Next, logarithmic conversion processing 9004 is executed. The logarithmic conversion process is a process of logarithmically converting the ratio between the radiation intensity without attenuation and the radiation intensity attenuated through the subject to obtain the attenuation rate a, and is represented by the following equation (1).

a (u, v) = In ((I0 (u, v)) / I (u, v)) = ∫μdt (1)

Here, a (u, v) is an attenuation factor at a position (u, v) on the radiation detector, and I0 (u, v) is detected by a radiation detection element in a channel (u, v) on the radiation detector. Radiation intensity with no attenuation (detector output), I (u, v) represents the radiation intensity with attenuation detected at the same position. Further, μ represents a linear attenuation coefficient depending on the material and radiation energy, and t represents a radiation transmission path.

  Image reconstruction is a process for obtaining the spatial distribution of the linear attenuation coefficient μ using the left side of Equation (1) as an input value. Subsequently, preprocessing 9005 is executed. In pre-processing 9005, a correction for a device having a variation or a defect among a large number of detection elements or a device-dependent correction is performed. This pre-processing 9005 may be performed before the logarithmic conversion processing 9004 according to circumstances.

  After the above processing, back projection operation processing 9006 is executed. The backprojection calculation process is a process of mapping (backprojecting) data corrected and converted so far into a two-dimensional or three-dimensional space. The DTS method described above corresponds to the movement and overlay processing of transparent image data. By this back projection operation, a two-dimensional tomographic image or a three-dimensional stereoscopic image (reconstructed image) is finally generated.

  Next, data used in the conventional method will be described with reference to FIG.

  FIG. 18 shows data stored in a conventional storage device. The air layer data 932 is measured in advance before imaging the subject. Using the air layer data 932 and the transmission image data 933, an attenuation factor 934 is obtained. That is, it is meaningless if the air layer data obtained in advance is measured in a state where the detector output is saturated, so the output of the X-ray generator is set so that the output of the detector does not saturate even when there is no subject. It is necessary to squeeze or reduce the gain of the amplifier circuit.

  Next, the relationship between the X-ray intensity and the detector output will be described with reference to FIG. In FIG. 19, the horizontal axis indicates the X-ray intensity, and the vertical axis indicates the output of the detector (the voltage obtained by amplifying the output of each channel with an amplifier).

  Since there is an upper limit on the amount of charge generated by X-rays that can be accumulated in each channel of the detector, and there is an upper limit on the amplified output of the two-dimensional detector and the amplifier circuit, the detector output is saturated at a certain voltage. An alternate long and short dash line 953 indicates a case where the gain of the amplifier circuit is high (case 1), and a solid line 954 indicates a case where the gain of the amplifier is set appropriately lower (case 2). The output of the detector saturates when it exceeds the measurement range 955 of the detector or amplifier circuit.

  Therefore, it is necessary to set the X-ray intensity and the gain of the amplifier circuit as in Case 2 in order to prevent the detector output from being saturated even in the air layer with the highest X-ray intensity (956). With this setting, the detector output I0 (957) at the time of air layer measurement can be suppressed to a saturation output or less.

  When an X-ray tube with an output of 225 kV, which is normally used for transmission imaging, is used, it is expected that the X-ray transmitted through the subject will be attenuated by about two orders of magnitude with an iron thickness of 2 cm. That is, in FIG. 19, the detector output I (959) corresponding to the X-ray intensity range 958 transmitted through the subject is at a level two orders of magnitude smaller than the output I0 (957) in the case of the air layer. . For this reason, there is a possibility that the S / N of the measurement data may decrease due to an increase in output fluctuation due to a decrease in the number of X-ray photons and an increase in error during AD conversion. A decrease in S / N of the transmission image data leads to a decrease in the image quality of the tomographic image.

  However, as described above, since it is necessary to determine the imaging conditions so that the output is not saturated in all the channels of the radiation detector even during imaging of the subject, the S / N of the measurement data may be reduced, and thus the tomographic image. There is a problem that it leads to a decrease in image quality.

Next, image reconstruction processing in the radiation tomography apparatus according to the present embodiment will be described with reference to FIGS.
FIG. 9 is a flowchart showing the processing contents of image reconstruction in the radiation tomography apparatus according to the embodiment of the present invention. FIG. 10 is an explanatory diagram of data used in the image reconstruction method in the radiation tomography apparatus according to the embodiment of the present invention. FIG. 11 is an explanatory diagram of the relationship between the X-ray intensity and the output of the detector in the image reconstruction method in the radiation tomography apparatus according to the embodiment of the present invention.

  First, the processing contents of image reconstruction according to the present embodiment will be described with reference to FIG.

  First, a condition input process 1001 for inputting calculation conditions such as input data names and calculation parameters is executed. Next, based on the input calculation conditions, the image reconstruction device 22 reads the transmission image data 51 captured by the radiation imaging device 11 from the storage device 31 and the channel constant data 75 are stored in the storage device. The channel constant data reading process 1003 to be read from is executed.

  In the present embodiment, the air constant data is not stored in the storage device in advance, but the channel constant data 75 is used. In the transmission image data reading process 1002 and the channel constant data reading process 1003, the first transmission image data obtained by photographing the part of the pipe that requires image reconstruction in order to check the state of the defective part of the pipe, The second transparent image data captured before and after the first transparent image data is read.

  Next, an air layer data calculation process 1004 for estimating the air layer data of each channel from the transmission image data is performed. This is a process of calculating the air layer data of each channel using the measurement data of the channel with the shield and the channel constant data 75. Details will be described later.

  Next, logarithmic conversion processing 1005 is executed. This process is a process for obtaining the attenuation rate a by logarithmically converting the ratio between the radiation intensity without attenuation (air layer data) and the radiation intensity attenuated through the subject as in the conventional process. However, data calculated in the air layer data calculation process 1004 is used as the air layer data.

  Subsequently, pre-processing 1006 is executed, back projection calculation processing 1007 is executed, and finally a two-dimensional tomographic image or a three-dimensional stereoscopic image (reconstructed image) is generated.

  Next, data used in the method of this embodiment will be described with reference to FIG.

  In this embodiment, the air constant data is not stored in the storage device in advance, but the channel constant 75 is used. FIG. 10 shows data stored in the storage device 31 in this embodiment. The two-dimensional radiation detector 2 has u1 × v1 number of channels. The attenuation rate data 55 is obtained from the channel constant 75 and the measured transmission image data 51.

  When the measured value I (u, v) is saturated, the attenuation factor is zero.

  Next, the relationship between the X-ray intensity and the detector output in the present embodiment will be described with reference to FIG. The horizontal axis of FIG. 11 indicates the intensity of X-rays entering the detector, and the vertical axis indicates the output of the detector (the voltage obtained by amplifying the output of each channel with an amplifier).

  The output characteristics of the detector (the gain of the amplifier circuit and the output current of the X-ray tube) are set to the maximum gain and the maximum current that do not saturate the detector output due to the X-ray intensity transmitted through the subject. The output characteristics 101 of each channel are as shown in FIG. The gain of the amplification circuit and the output of the X-ray tube are determined so that the detector output 104 when the X-ray level 103 transmitted through the pipe enters the detector is lower than the saturation level Imax of the detector output and as large as possible. To do. In particular, the X-ray tube output is set as large as possible, the number of X-ray photons is increased, and the gain of the amplifier circuit is set as low as possible to reduce circuit noise.

  In the following description, for simplicity, the reference channel is 1 channel (u1, v1), and I_ref = I (u1, v1). The reference channel (u1, v1) with the shield is exposed to the X-ray 105 that does not pass through the subject. However, since the X-ray is shielded by the shield, the reference channel output I_ref (106) is the saturation level of the detector output. It does not exceed Imax and can always be measured during imaging. In addition, the thickness of the shield is set and installed.

  The shield may be fixed at a certain thickness, but if the subject is thick and the gain of the amplification circuit needs to be increased, it is thick. Is thin and can be removable so that it can be changed. Since the shield is preferably a substance having a high X-ray shielding capability, tungsten or an alloy thereof is preferable. Of course, materials with high density and atomic number such as lead can be used.

A normal channel other than the reference channel naturally saturates the detector output when exposed to X-rays (air layer) that does not pass through the subject, but the estimated value I0 (u, v) of the air layer output is The following equation (2) is obtained.

I0 (u, v) = k (u, v) × I_ref (2)

In FIG. 11, this estimated value 107 is shown.

Accordingly, the attenuation rate a (u, v) of the channel (u, v) can be obtained by the following equations (3) and (4).

Here, if I (u, v) <Imax,

a (u, v) = In (k (u, v) × I_ref / I (u, v)) (3)

That is, the attenuation rate a (u, v) of each channel (u, v) is obtained by multiplying the channel constant k (u, v) by the output value I_ref of the reference channel covered with the shield, Obtained by dividing by the output I (u, v) in FIG.

If I (u, v) = Imax,

a (u, v) = 0 (4)

It becomes. That is, when the measured value I (u, v) is saturated, the attenuation factor is zero.

  Here, a (u, v) is an attenuation factor at a position (u, v) on the radiation detector, k (u, v) is a channel constant of a channel (u, v) on the radiation detector, and I (u , V) indicates the radiation intensity detected at the same position, I_ref indicates the radiation intensity of the reference channel with a shield, and Imax indicates the saturation output of the detector.

  In the description so far, the reference channel is one channel, but since the size of each channel of the two-dimensional radiation detector is about 0.2 to 0.5 mm square, a shield is installed in one channel. It is difficult. In addition, since the X-ray dose incident on the channel is reduced by about two digits due to the shield, the average processing is performed from the measurement of a plurality of channels. Considering the practical size of the shield to be installed, 100 channels can be shielded even with a 5 mm square shield, so that the average processing can maintain the output fluctuation of the reference channel equivalent to that measured without the shield.

  As described above, according to the present embodiment, both the high radiation dose necessary for increasing the S / N of measurement data and the high gain setting of the measurement circuit are compatible, and the X-ray transmitted through the subject has a good S / N. Since it can be measured, a tomographic image can be reconstructed with high image quality from radiation transmission image data.

Next, display examples in the radiation tomography apparatus according to the present embodiment will be described with reference to FIGS.
12 and 13 are explanatory diagrams of display examples of a plurality of transmission image data in the radiation tomography apparatus according to the embodiment of the present invention. FIG. 14 is an explanatory diagram of a display example of a three-dimensional stereoscopic image in the radiation tomography apparatus according to the embodiment of the present invention.

  12 and 13 show an example of a screen in which a plurality of transmission image data is displayed on the screen of the determination device 24. FIG. In this figure, when the transparent image reading button 60 is pressed, the transparent image data is read from the transparent image data storage device 31 and a plurality of transparent image data 51 is displayed on the screen as a moving image. As shown in FIG. 7, the transmission image data 51 is an image in which the subjects 10c and 10d positioned between the radiation source 1 and the radiation detector 2 are superimposed in the depth direction. Therefore, it is possible to display the pipe thinning portion 63 in a grayscale display in the transmission image data 51 without reconstructing a two-dimensional image or a three-dimensional image of the pipe. In addition, since the area where the two-dimensional image or the three-dimensional image is reconstructed can be narrowed down by screening the location of the pipe where the defect such as thinning occurs with the transmission image, the calculation amount of the image reconstruction is reduced. Is possible. Furthermore, since it is not necessary to reconstruct the entire length of the pipe, the storage capacity of the reconstructed image storage device that stores a two-dimensional image or a three-dimensional image can be reduced.

  When the operator confirms the transmission image data 51 and finds the thinned portion 63 of the pipe, the operator presses the button 61 for stopping the moving image to confirm the thinned portion 63 in the three-dimensional image of the pipe. Then, a determination result is input by pressing a button 62 for determining whether image reconstruction is necessary.

  If it is determined that image reconstruction is necessary, the screen transitions to FIG. On this screen, the reconstruction area 65 is designated by the pointer 64 using an input device such as a mouse connected to the PC, and the button 66 is pressed to execute the calculation. By depressing the button 66, a command for image reconstruction is issued to the image reconstruction device 22. When the reconstruction area 65 is designated, it may be designated by a rectangle so as to surround the pipe thinning portion 63.

  FIG. 14 shows a result of displaying a two-dimensional tomographic image or a three-dimensional stereoscopic image on the screen for the piping portion corresponding to the reconstruction area 65 of the transmission image. By reconstructing a two-dimensional tomographic image or a three-dimensional stereoscopic image, the three-dimensional shape of the pipe thinning portion 63 can be easily confirmed. In this way, only a portion where a defect such as a thinning of the pipe is suspected in the transmission image data can be confirmed with a two-dimensional tomographic image or a three-dimensional stereoscopic image.

  As described above, according to the present embodiment, X-rays transmitted through a subject can be measured with good S / N, so that a tomographic image can be reconstructed from radiation transmission image data with high image quality, and can be installed in a plant or the like. A high-quality tomographic image can be obtained in non-destructive inspection by radiography of pipes and the like.

Next, the configuration and operation of a radiation tomography apparatus according to another embodiment of the present invention will be described with reference to FIGS. 15 and 16.
FIG. 15 is a block diagram showing a system configuration of a radiation tomography apparatus according to another embodiment of the present invention. The same reference numerals as those in FIG. 1 denote the same parts. FIG. 16 is an explanatory diagram of data used in an image reconstruction method in a radiation tomography apparatus according to another embodiment of the present invention.

  In the present embodiment, as shown in FIG. 1, a shield is not provided on the detector panel 71 of the radiation detector 2, but the monitor detector 81 is arranged in the vicinity of the radiation detector 2 as shown in FIG. 15. Provided. The monitor detector 81 is installed at a position where the radiation generated from the radiation source 1 is always directly irradiated.

  As described with reference to FIG. 1, the radiation detector 2 is a two-dimensional radiation detector in which a CCD (Charge Coupled Device) panel or a TFT (Thin Film Transistor) panel is installed behind the scintillator surface that emits light when radiation is incident. It is. Therefore, the dynamic range of each channel of the radiation detector 2 is narrow.

  On the other hand, the monitor detector 81 uses a radiation detector such as a semiconductor detector or an ion chamber whose output is not saturated with a current output. In the monitor detector 81, for example, silicon having a depletion layer is placed behind the scintillator surface that emits light when radiation is incident. The monitor detector 81 is larger than the size of each channel of the radiation detector 2. Therefore, the monitor detector 81 has a wider dynamic range than each channel of the radiation detector 2. That is, the relationship between the X-ray intensity in the monitor detector 81 and the output of the detector has the same characteristics as the estimated value 107 of the air layer output shown in FIG.

  The output of the monitor detector 81 is amplified and A / D converted by the detection circuit 82 and stored in the storage device 31 in synchronization with the transmission of the transmission image data 51 by the control device.

  Next, data stored in the storage device 31 in this embodiment will be described with reference to FIG.

  In the present embodiment, measurement data (I_ref) 77 of the monitor detector 81 is stored in addition to the channel constant data 76 and the transmission image data 51, and the attenuation rate data 78 is the channel constant data 76 and the measurement data of the monitor detector 81. 77 and the transmission image data 51. The channel constant data is determined based on the output of the monitor detector.

  The channel constant data 76 is a value calculated as (I0 (u, v) / I_ref) from the measurement data (I_ref) of the monitor detector 81 and the air layer output I0 (u, v) of other channels. is there.

  The attenuation rate a (u, v) of each channel (u, v) is obtained by the above-described equation (3). That is, the attenuation rate a (u, v) of each channel (u, v) is obtained by multiplying the channel constant k (u, v) by the output value I_ref of the reference channel, which is measurement data of the monitor detector 81, and other than the reference channel. Obtained by dividing by the output I (u, v) in each channel is obtained by logarithmic conversion.

  When the measured value I (u, v) is saturated, the attenuation factor is zero.

  The initial setting flow is the same as that shown in FIG. 3 except that the channel constant data is determined based on the output of the monitor detector, and the photographing flow and the image processing flow are also the same, and the description thereof will be omitted. . The reconstruction process is the same except that I_ref is the monitor detector output.

  According to the present embodiment, the X-ray transmitted through the subject can be measured with good S / N while achieving both the high radiation dose necessary for increasing the S / N of the measurement data and the high gain setting of the measurement circuit. A tomographic image can be reconstructed with high image quality from image data.

  Further, even if the gain of the amplification circuit 72 of the radiation detector 2 is changed, the labor for changing the thickness of the shield can be saved, so that it can be realized more easily.

  In the above description, each of the inspection systems translates the radiation source and the radiation detector in the same direction with respect to the subject. However, either the radiation source or the radiation detector is moved, rotated, or reversed. The same effect can be obtained even with an inspection system that translates in the direction.

  In addition, even in a system that obtains a tomographic image by placing a subject on a turntable and rotating it like an industrial X-ray CT apparatus, a shield is placed on a detector that is not always shielded by the subject, The same effect can be obtained by installing another monitor detector in a position where it is not always shielded, and obtaining the air layer data of the channel to be written from the channel constant and the output information of the monitor detector.

In the present invention, not only piping installed in a power plant but also large structures such as aircraft wings can be efficiently inspected with radiation with high image quality.

It is a block diagram which shows the system configuration | structure of the radiation tomography apparatus by one Embodiment of this invention. It is a top view which shows the structure of the radiation detector used for the radiation tomography apparatus by one Embodiment of this invention. It is a flowchart which shows the inspection method in the radiation tomography apparatus by one Embodiment of this invention. It is a perspective view which shows the specific structure of the radiation tomography apparatus by one Embodiment of this invention. It is explanatory drawing of the imaging state in the radiation tomography apparatus by one Embodiment of this invention. It is explanatory drawing of the collection range of the transmission image data required for the image reconstruction in the radiation tomography apparatus by one Embodiment of this invention. It is explanatory drawing of the principle of the image reconstruction by DTS method in the radiation tomography apparatus by one Embodiment of this invention. It is explanatory drawing at the time of applying Limited Angle reconstruction image technique like DTS method in the radiation tomography apparatus by one Embodiment of this invention to imaging of piping. It is a flowchart which shows the processing content of the image reconstruction in the radiation tomography apparatus by one Embodiment of this invention. It is explanatory drawing of the data used with the image reconstruction method in the radiation tomography apparatus by one Embodiment of this invention. It is explanatory drawing of the relationship between the X-ray intensity in the image reconstruction method in the radiation tomography apparatus by one Embodiment of this invention, and the output of a detector. It is explanatory drawing of the example of a display of several transmission image data in the radiation tomography apparatus by one Embodiment of this invention. It is explanatory drawing of the example of a display of several transmission image data in the radiation tomography apparatus by one Embodiment of this invention. It is explanatory drawing of the example of a display of the three-dimensional solid image in the radiation tomography apparatus by one Embodiment of this invention. It is a block diagram which shows the system configuration | structure of the radiation tomography apparatus by other one Embodiment of this invention. It is explanatory drawing of the data used with the image reconstruction method in the radiation tomography apparatus by other embodiment of this invention. It is a flowchart which shows the processing content of the conventional image reconstruction. It is explanatory drawing of the data used with the conventional image reconstruction method. It is explanatory drawing of the relationship between the X-ray intensity in the conventional image reconstruction method, and the output of a detector.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Radiation source, 2 ... Radiation detector, 3 ... Scanner apparatus, 3a ... C-shaped arm, 3b ... Guide rail, 3c ... Support leg, 5 ... Radiation, 10 ... Piping, 10a ... Object, 10c ... Subject, DESCRIPTION OF SYMBOLS 10d ... Subject, 11 ... Radiation imaging device, 12 ... Control arithmetic device, 22 ... Image reconstruction device, 23 ... Image measuring device, 24 ... Judgment device, 26 ... Input device, 27 ... Storage device, 28 ... Attenuation amount calculation Program, 29 ... arithmetic device, 30 ... storage device, 31 ... storage device, 41 ... arithmetic device, 42 ... storage device, 51 ... transmission image data, 52 ... reconstructed image, 55, 78 ... attenuation rate data, 71 ... detection Panel 72 ... amplifier circuit 73 ... AD converter circuit 74 ... shielding body 75 ... channel constant

Claims (4)

A radiation imaging apparatus having a radiation source and a two-dimensional radiation detector arranged opposite to each other with a subject interposed therebetween, and radiation emitted from the radiation source and transmitted through the subject are detected by the radiation detector. A radiation tomography apparatus having a control arithmetic unit for constructing a tomographic image of the subject from a set of transmission data of the subject,
The radiation imaging apparatus includes a shield that shields radiation irradiated from the radiation source in a part of the plurality of channels of the radiation detector,
A channel in which radiation is shielded by the shield is a reference channel,
The control arithmetic unit is
The output I0 (u, v) of each channel other than the reference channel of the radiation detector in a state where the subject is not installed between the radiation source and the radiation detector, and the radiation detector A storage device for storing a channel constant k (u, v) having a ratio (I0 (u, v) / I_ref) to the output I_ref of the reference channel;
The channel constant k (u, v) is multiplied by the output value I_ref of the reference channel, and the output in each channel other than the reference channel in a state where the subject is installed between the radiation source and the radiation detector. Radiation comprising: an image reconstruction device that calculates an attenuation factor a (u, v) of each channel (u, v) as a logarithmically converted one divided by I (u, v) Tomography equipment. A radiation tomography method for constructing a tomographic image of a subject from a set of transmission data of the subject obtained by arranging a radiation source and a radiation detector facing each other across the subject,
A reference channel in which radiation irradiated from the radiation source is shielded by a shielding body among the channels of the radiation detector in a state where the subject is not installed between the radiation source and the radiation detector. A channel constant k (u, v) consisting of a ratio (I0 (u, v) / I_ref) between the output I_ref of the radiation detector and the output I0 (u, v) of a channel other than the reference channel of the radiation detector is calculated. ,
The channel constant k (u, v) is multiplied by the output value I_ref of the reference channel, and the output in each channel other than the reference channel in a state where the subject is installed between the radiation source and the radiation detector. What is divided by I (u, v) is logarithmically transformed to calculate an attenuation rate a (u, v) of each channel (u, v). A radiation imaging apparatus having a radiation source and a two-dimensional radiation detector arranged opposite to each other with a subject interposed therebetween, and radiation emitted from the radiation source and transmitted through the subject are detected by the radiation detector. A radiation tomography apparatus having a control arithmetic unit for constructing a tomographic image of the subject from a set of transmission data of the subject,
The radiation imaging apparatus includes a monitor detector that is disposed at a position where radiation irradiated from the radiation source is always irradiated and has a dynamic range wider than each channel of the radiation detector,
The control arithmetic unit is
Ratio of output I0 (u, v) of each channel of the radiation detector and output I_ref of the monitor detector in a state where the subject is not installed between the radiation source and the radiation detector A storage device for storing a channel constant k (u, v) consisting of (I0 (u, v) / I_ref);
The channel constant k (u, v) is multiplied by the output value I_ref of the monitor detector, and the output I (u) in each channel when the subject is placed between the radiation source and the radiation detector. , V), and an image reconstruction device that calculates an attenuation factor a (u, v) of each channel (u, v) as a logarithmically converted one. . A radiation tomography method for constructing a tomographic image of a subject from a set of transmission data of the subject obtained by arranging a radiation source and a radiation detector facing each other across the subject,
When the subject is not installed between the radiation source and the radiation detector, the output I0 (u, v) of each channel of the radiation detector and the radiation irradiated from the radiation source are always A channel constant k (u, v) comprising a ratio (I0 (u, v) / I_ref) to the output I_ref of the monitor detector which is arranged at the irradiation position and has a dynamic range wider than each channel of the radiation detector. )
The channel constant k (u, v) is multiplied by the output value I_ref of the monitor detector, and the output I (u) in each channel in a state where the subject is placed between the radiation source and the radiation detector. , V), and calculating the attenuation rate a (u, v) of each channel (u, v) as a logarithmically converted one.

Images