JP2598037B2 - Tomographic imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a tomographic imaging apparatus used for nondestructive inspection and the like.

(Prior Art) As a tomographic imaging apparatus, a specific cross section of an object to be inspected is irradiated with X-rays from all around by an X-ray generator as a radiation source, and an X-ray detector transmits X-rays in each direction. The so-called X-ray CT, which obtains projection data by measuring the dose and obtains a tomographic image of a specific section of the object from the projection data
Scanners are common. From the tomographic image obtained by the CT scanner, information such as the cross-sectional shape of the object to be inspected and the presence or absence of a defect can be obtained. Therefore, the tomographic image is widely used for nondestructive inspection and the like for industrial use.

By the way, as the X-ray detector, a plurality of radiation detecting elements in which a scintillator that emits light instantaneously upon incidence of X-rays and a photoelectric conversion element that converts light emitted by the scintillator into an electric signal are arranged. In general, a shield plate is interposed between detection elements to prevent the influence of scattered X-rays, and projection data of the number of channels corresponding to the number of detection elements can be obtained by one X-ray irradiation. It has become.

However, in an industrial X-ray CT scanner, a high-energy X-ray tube of, for example, 420 [kV] may be used as an X-ray source, and an X-ray detector has a detection element of, for example, 512 [k] to obtain high resolution. ch], and the thickness of the shielding plate provided between the elements is reduced. Therefore, a so-called crosstalk phenomenon occurs in which high-energy X-rays scattered in the scintillator pass through the shielding plate and enter an adjacent scintillator. This phenomenon causes deterioration in image quality such as a decrease in resolution of an output image and an increase in image artifacts at an edge portion.

(Problems to be Solved by the Invention) As described above, in the conventional tomographic imaging apparatus,
The crosstalk phenomenon in which scattered radiation in the scintillator passes through the shielding plate and enters the adjacent scintillator may cause deterioration in image quality such as a decrease in resolution of an output image and an increase in image artifacts at an edge portion.

Therefore, an object of the present invention is to provide a tomographic imaging apparatus capable of removing an influence on an output image due to a crosstalk phenomenon and obtaining a high-resolution tomographic image with good image quality.

[Configuration of the Invention] (Means for Solving the Problems) According to the present invention, a specific cross section of an object to be inspected is irradiated with radiation in a fan shape from a plurality of directions at a predetermined spread angle, and the radiation from each direction of the specific cross section is irradiated. In a tomographic imaging apparatus configured to collect projection data and output a tomographic image by a radiation transmittance distribution of a specific cross section by image reconstruction processing, based on information about scattered radiation between radiation detecting elements in a radiation detector. And data correction means for correcting the collected radiation projection data.

(Operation) By taking such a measure, an influence component due to scattered radiation between the radiation detection elements is corrected for the collected radiation projection data, which leads to an improvement in image quality.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing an embodiment in which the present invention is applied to an X-ray CT scanner. As shown in FIG.
An X-ray source 12 for irradiating a fan-shaped X-ray beam 11 with a predetermined spread angle, and a plurality of X-ray detectors 13 capable of receiving all of the X-ray beams 11 from the X-ray source 12 are rotated by a rotation mechanism 14. And a drive frame 15 rotatably moved up and down by an elevating mechanism (not shown). The drive frame 15 is placed in a predetermined position by arranging a test object 17 in an imaging region 16 of the X-ray beam 11 in the frame opening 15a. Make one rotation at the position, ie X
By rotating the radiation source 12 and the X-ray detector 13 once, the X-ray detector 13 obtains X-ray projection data from each direction with respect to a specific section of the object 17 to be inspected.

The data collection unit 18 collects X-ray projection data from multiple directions detected by the X-ray detector 13, and the collected data is sent to a CPU (central processing unit) 19. The CPU 19 has a built-in arithmetic circuit and the like, performs a correction processing function of performing various corrections on the X-ray projection data collected by the data collection unit 18, and converts the corrected data into a tomographic image based on an X-ray transmittance distribution according to a reconstruction algorithm. It has a reconfiguration processing function for performing reconfiguration. The CPU 19 has a memory unit for storing a reconstruction algorithm, correction data, and the like.
20 and the control console unit 21 are connected. The control console unit 21 includes a CRT display 22 for displaying a tomographic image based on input information from the CPU 19 or the outside, a mechanism control unit 23 for controlling the driving of the rotating mechanism 14 and the elevating mechanism of the drive frame 15, and X It controls the operation of the X-ray control unit 24 for controlling the X-ray emission from the radiation source 12.

The radiation detector 13 includes, for example, a plurality of detection element units in which a plurality of radiation detection elements in which a scintillator and a photodiode as a photoelectric conversion element are collimated by a collimator as a shielding plate are fan-shaped on a detector base. FIGS. 2A and 2B show a side view and a top view of the detection element unit. In the figure, reference numeral 31 denotes a unit base, on which a plurality of collimators 32 and a signal processing unit 33 are fixed. The collimators 32 are arranged at predetermined intervals, and an X-ray detection element including a scintillator 34 and a photodiode 35 is mounted in each gap. Each photodiode 35 is connected to a lead 36
Is connected to the signal processing unit 33, and the signal processing unit
33 is the data collection unit via a connector (not shown)
Connected to 18. When the X-ray beam 11 reaches the scintillator 34 between the collimators 32 and causes the scintillator to emit light, this light is converted into an electric signal by the photodiode 35 and reaches the signal processing unit 33 via the lead 36, Further, the data is collected by the data collection unit 18.

Thus, the CPU 19 is configured to execute the operation shown in the flowchart of FIG. That is, when the X-ray projection data collected by the data collection unit 18 is received, the X-ray beam 11
Is corrected (offset correction) for the output of the detector that does not reach the position, and then crosstalk correction described later is performed. Next, if the crosstalk correction output is logarithmically converted, a correction for the intensity change of the X-ray beam 11 (reference correction) is performed, and a correction for the sensitivity using air data (air correction) is performed. Correction for curing (beam hardening correction) is performed, and the correction processing ends. Next, as image reconstruction processing B, convolution conversion is performed on the correction data subjected to various corrections by the correction processing A,
Further, a back-projection (back-projection) calculation is performed to obtain a tomographic image based on the X-ray transmittance distribution.
Control console unit 21 to display on T display 22
Is controlled.

Here, the crosstalk correction is performed as follows. That is, the actual output of each radiation detecting element in the radiation detector (the number of channels = N) 13 after the offset correction is set to P ₁ , P ₂ ,..., _PN, and no crosstalk phenomenon occurs between the respective detecting elements. In this case, ideal outputs after offset correction are Q ₁ , Q ₂ ,..., Q _N. Also, let αji be the proportion of scattered X-rays that pass through the collimator 32 from an arbitrary radiation detection element i to another radiation detection element j and enter. In this case, since the scattered X-rays to the portions other than the adjacent radiation detection element are extremely weak and can be ignored, the relationship of the following equation (1) is established between the actual output P and the ideal output Q.

In the equation (1), αji is a value obtained by actual measurement in advance, and P _{1 to} P _N are also detection outputs. Therefore, the unknowns Q _{1 to} Q _N can be obtained by solving the N simultaneous linear equations of the above equation (1).

The following expression (2) is obtained by expressing the expression (1) in a matrix format.

Equation (2) is simplified Can be expressed as Therefore, And ask for Becomes Here, α is usually a small value (several percent or less). Can be approximated as in the following equation (5).

Therefore, by calculating the above equation (5), it is possible to obtain an ideal output Q in which the influence of scattered X-rays transmitted through the collimator and removed from adjacent detection elements is removed for each X-ray detection element.

The above αji is obtained in advance by the method shown in FIG. That is, the X-ray source 12 irradiates the X-ray beam 11 with the X-ray detecting element except for the X-ray detecting element other than the X-ray detecting element, and the detecting elements i and i
Measure the output of +1. Here, detection elements i and i +
If the offset-corrected outputs of 1 are P _i and P _{i + 1} , α
_{i + 1, i} is obtained by α _{i + 1, i} = P _{i + 1} / P _i (6) By performing the above-described measurement for all combinations of the detection elements, each α is obtained. In practice, however, it may be obtained for one detection element unit.

As described above, a so-called crosstalk phenomenon occurs in which the X-ray beam 11 arriving at the scintillator 34 passes through the collimator 32 by scattering and enters the adjacent scintillator between the X-ray detection elements constituting the X-ray detector 13. However, in the present embodiment, the error of the output of each detection element due to the crosstalk phenomenon is corrected with high accuracy by the crosstalk correction as one of the correction processing functions in the CPU 19, and the crosstalk corrected data is image-corrected. Reconstruction processing is performed to obtain a tomographic image at a specific cross section of the subject.

Therefore, according to the present embodiment, the influence of the crosstalk phenomenon can be reliably removed by a simple correction operation, the resolution of the output image can be increased, and the image artifact at the edge portion can be reduced. In addition, since the output error due to the crosstalk phenomenon can be corrected, the energy of the X-ray beam 12 can be increased and the X-ray detector 13 can be densely mounted, so that the practicality can be improved.

In the above embodiment, the case where the crosstalk phenomenon between the detection elements is not uniform has been described. However, when the crosstalk phenomenon is uniform, it is possible to perform the crosstalk correction according to the flowchart of FIG. Become. That is, projection data τ (t) on which offset correction, reference correction, air correction, and BHC correction have been performed, where t is an X-ray detection element.
Is obtained, and is subjected to exponential conversion (formula (7)), and further Fourier transformed (formula (8)).

x (t) = exp [−τ (t)] (7) X (ω) = F (x (t)) (8) Then, from an arbitrary X-ray detecting element by the uniformized crosstalk phenomenon Fourier transform is performed on a crosstalk function q (t) corresponding to the ratio of scattered X-rays transmitted to an adjacent X-ray detection element (Equation (9)).

Q (ω) = F [q (t)] (9) Then, the output X (ω) after the Fourier transform has a coefficient [1 / Q
(Ω)] to remove the output component due to the crosstalk phenomenon (Equation (10)).

X ′ (ω) = X (ω) / Q (ω) (10) Thereafter, the output after the coefficient multiplication is subjected to inverse Fourier transform (formula (11)), and logarithmic transform is further performed to perform crosstalk correction. The processing is terminated (formula (12)), and the process proceeds to image reconstruction processing.

x ′ (t) = F ⁻¹ [X ′ (ω)] (11) τ ′ (t) = − ln (x ′ (t)) (12) When the crosstalk phenomenon is uniformed By performing the above-described crosstalk correction, the influence on the output image due to the crosstalk phenomenon can be removed, and the image quality can be improved. However, when the ratio of the uniformized crosstalk phenomenon is small, the exponential conversion and the logarithmic conversion may be omitted.

In the above embodiment, the case where the present invention is applied to an X-ray CT scanner is exemplified. However, it is needless to say that the present invention can be applied to a tomographic image pickup apparatus using other radiation such as γ-ray. Further, the configuration of the scanner unit 10 fixes the radiation source and the radiation detector,
The object to be inspected may be placed on a drive table and moved up and down and rotated. In addition, it goes without saying that various modifications can be made without departing from the spirit of the present invention.

[Effects of the Invention] As described above in detail, according to the present invention, it is possible to remove the influence of the crosstalk phenomenon on an output image and obtain a high-resolution and high-quality tomographic image. Can be provided.

[Brief description of the drawings]

1 to 4 are views showing an embodiment of the present invention, wherein FIG. 1 is a configuration diagram of an X-ray CT scanner, FIG. 2 is a configuration diagram of a detector unit in an X-ray detector, FIG. 3 is a flowchart showing the operation of the CPU, FIG. 4 is a diagram for explaining the crosstalk correction, and FIG. 5 is a flowchart showing a modification of the crosstalk correction in the present invention. 11 X-ray beam, 12 X-ray source, 13 X-ray detector,
18 Data collection unit, 19 CPU, 22 CRT display, 32 Collimator, 34 Scintillator, 35 Photodiode, 40 Shielding member.

Claims (1)

(57) [Claims] 1. A radiation source for irradiating a specific section of a test object with radiation in a fan shape at a predetermined spread angle, and a scintillator disposed between a shielding plate disposed opposite to the radiation source and fixed at a predetermined interval. A radiation detector in which a plurality of radiation detection elements each including a photoelectric conversion element are arranged, and a data collection unit that collects radiation projection data from each direction of a specific cross section of the object detected by the radiation detector. For the radiation projection data collected by the data collection means, the arbitrary radiation based on the ratio of scattered X-rays transmitted from the arbitrary radiation detection element in the radiation detector to another radiation detection element through the shielding plate and incident. Data correction means for performing correction by linearly combining the data of the detection elements with the data of the other radiation detection elements, and the data corrected by the data correction means Tomography apparatus characterized by comprising a tomographic image output means for outputting a tomographic image by radiation transmission distribution of the specific section is subjected to image reconstruction processing over data.