JP2004037267A - Computed tomography - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a computed tomography for always accurately estimating a projection position in the rotary center axis of a specimen without using any exclusive phantom or the like and hence without being affected by the manufacturing precision of the phantom. <P>SOLUTION: By using X-ray transmission data for 360° in an arbitrary specimen W and by utilizing that the degree of matching of data that differ by 180° is high at a projecting position in the rotation axis, an element corresponding to the projection position onto the light reception surface of an X-ray detector 2 in the rotary axis Ra of the specimen W is obtained. By using the X-ray transmission data for 360° with an arbitrary object as the specimen, the projection position of the rotational center axis can be estimated stably and accurately, thus eliminating an estimation error in the projection position of the rotational center axis caused by manufacturing errors in such a component as the phantom and achieving stable high quality in sectional images. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a computer tomography apparatus, and more particularly, to a computer tomography apparatus that rotates an object between an X-ray source and an X-ray detector.
[0002]
[Prior art]
For example, in an X-ray CT apparatus used for industrial destructive inspection, that is, a computed tomography apparatus using X-rays, generally, an X-ray (fan beam or cone beam) having at least one direction is generated. An X-ray source and an X-ray detector in which a plurality of elements are arranged in the direction in which the X-ray spreads are arranged opposite to the X-ray source. A configuration is adopted in which a turntable that rotates around a rotation axis that is orthogonal to both the axial direction and the arrangement direction of the X-ray detector elements is employed.
[0003]
There is also a configuration in which an X-ray source and an X-ray detector facing the X-ray source are integrally rotated around a stationary subject instead of rotating the subject.
[0004]
Then, while rotating the subject, or rotating the X-ray source and the X-ray detector, the intensity data of the X-rays transmitted through the subject at predetermined small angles are converted into the X-ray detector data for 360 degrees. A tomographic image of the subject along a plane orthogonal to the rotation axis is reconstructed using the X-ray transmission data obtained from the element output.
[0005]
Here, in reconstructing a tomographic image using the X-ray transmission data, the projection position of the rotation center axis of the subject on the light receiving surface of the X-ray detector (hereinafter referred to as the rotation center axis projection position) is used. Without accurate estimation, high quality of the obtained tomographic image cannot be achieved.
[0006]
Conventionally, as a method of estimating the rotation center axis projection position, a dedicated phantom having a structure such as passing a wire made of a material having a high X-ray absorptivity such as tungsten in an acrylic pipe is prepared, and the phantom is turned on a turntable. Mount on the sample stage to give rotation, or mount the phantom on the stationary sample stage, rotate the X-ray source and X-ray detector, and detect X-rays transmitted through the phantom with the X-ray detector From the X-ray transmission data for 360 degrees obtained as a result, the point where the wire is shifted leftmost and the point shifted right are detected, and the center point between these two points is estimated as the rotation axis center axis projection position. I was
[0007]
[Problems to be solved by the invention]
However, according to the conventional method of estimating the rotation center axis projection position described above, the manufacturing error of the dedicated phantom directly affects the estimation result of the rotation center axis projection position. There is a problem that is affected.
[0008]
The present invention has been made in view of such a situation, and an arbitrary object is mounted on a turntable to give rotation, or an X-ray source and an X-ray detector are rotated without using a dedicated phantom or the like. By collecting the X-ray transmission data in this way, it is possible to accurately estimate the projected position of the rotation center axis, and thus to prevent the quality of the tomographic image from being affected by the quality of the phantom, etc. It is an object of the present invention to provide a computer tomography apparatus capable of obtaining a high quality tomographic image.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, a computed tomography apparatus of the present invention includes an X-ray source and a plurality of elements arranged in a direction orthogonal to an X-ray optical axis from the X-ray source. An arrayed X-ray detector, a sample stage for abutting an object arranged between the X-ray source and the X-ray detector, and an object on the sample stage, the X-ray optical axis and The sample stage is rotated or the X-ray source and the X-ray detector are rotated so as to rotate relative to the element about a rotation axis orthogonal to both the X-ray optical axis and the arrangement direction of the elements. Tomographic images obtained by slicing a subject on a plane perpendicular to the rotation axis using rotational driving means for rotating the device and transmitted X-ray data detected by irradiating X-rays while driving the rotational driving means. In a computed tomography apparatus having a reconstruction operation means for reconstructing, By using 360-degree X-ray transmission data of a desired subject, a rotation center axis estimating unit utilizing the fact that the degree of coincidence between data having a rotation angle difference of 180 degrees is high in an element corresponding to the rotation center axis is described. Characterized by having.
[0010]
According to the present invention, when an object is rotated by 180 degrees relative to an X-ray source and an X-ray detector, a projection position of a point at an arbitrary position on the object on a light receiving surface of the X-ray detector Moves on the opposite side before and after the rotation about the projection position of the rotation axis, and uses only the element corresponding to the projection position of the rotation axis to completely match the X-ray fluoroscopic data that differs by 180 degrees. It is.
[0011]
That is, when the subject is rotated by 180 degrees relative to the X-ray source and the X-ray detector, an element located at a position distant from the projection position of the rotation axis has a position on the subject that has been projected before the rotation. The element corresponding to the projected position of the rotation axis has an element corresponding to the projected position of the rotation axis on the object that has been projected before the rotation when the object is relatively rotated by 180 degrees. Is projected as it is (only X-rays pass from the opposite side from before rotation).
[0012]
Therefore, for each element, the degree of coincidence (that is, the degree of coincidence of X-ray intensities) between X-ray transmission data in which the rotation angles of the subject are different from each other by 180 degrees is calculated, and the degree of coincidence is maximized or maximized. By determining an element exhibiting a value, the projected position of the rotation axis can be accurately estimated.
[0013]
According to such a method of the present invention, it is not necessary to use a dedicated phantom or the like as an object used for estimating the projected position of the rotation center axis, so that any object can be used. The quality of the tomographic image is not degraded due to the manufacturing error.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a configuration diagram of an embodiment of the present invention, in which a schematic diagram showing a mechanical configuration of a main part and a block diagram showing an electrical functional configuration are shown together.
[0015]
The X-rays generated by the X-ray source 1 are converted into a fan-shaped beam B via the collimator 11 and emitted in the direction (x direction) toward the X-ray detector 2 which is disposed opposite. The X-ray detector 2 has a structure in which a plurality of elements are arranged in an arc shape in the X-ray spreading direction (y-direction).
[0016]
Between the X-ray source 1 and the X-ray detector 2, a direction (z-direction) orthogonal to both the x-direction, which is the optical axis direction of the X-ray, and the y-direction, which is the arrangement direction of the elements of the X-ray detector 2. A turntable 3 that rotates about a rotation axis Ra along the axis is arranged, and the subject W is mounted on the turntable 3 and is rotated. The turntable 3 is mounted on an xyz table 31 that can move in the x, y, and z directions.
[0017]
The output of each element of the X-ray detector 2 is digitized by the data sampling circuit 4 every time the subject W is rotated by a small angle by the turntable 3, and is taken into the arithmetic unit 5. That is, the arithmetic unit 5 captures 360-degree X-ray transmission data of the subject W for each element of the X-ray detector 2. The arithmetic unit 5 includes a preprocessing unit 51 that performs gamma correction, image distortion correction, sensitivity correction of each element, logarithmic conversion, and the like of the X-ray transmission data from the data sampling circuit 4, and an X-ray after the preprocessing. In addition to a reconstruction operation unit 52 that performs a reconstruction operation of a tomographic image using transmission data and outputs the result to the display 6 or the like, a rotation center axis estimation unit 53 is provided. The projection position of the rotation axis Ra of the table 3 on the light receiving surface of the X-ray detector 2 is estimated and calculated, and the estimation result is used for a reconstruction calculation of a tomographic image in the reconstruction calculation unit 52.
[0018]
The rotation center axis estimating unit 53 projects the rotation axis Ra on the light receiving surface of the X-ray detector 2 by the following calculation using 360-degree X-ray transmission data from each element of the X-ray detector 2. Estimate the position.
[0019]
As illustrated in FIG. 2, each element (hereinafter, referred to as a channel) number i (i = 1 to n) of the X-ray detector 2 is taken on the horizontal axis, and the rotation angle φ ( (0 ° to 360 °), and consider a sinogram in which projection data P for 360 degrees of all channels are arranged. FIG. 2 illustrates a sinogram in the case of a subject in which a wire made of a material having a high X-ray absorptivity such as tungsten is passed through the above-mentioned acrylic pipe. In such a sinogram, for each channel i = 1 to n, the ratio of P [i, φ] to P [n, φ + 180 °] (do not exceed 1.0 using the larger one as the denominator) Calculation is performed in the range of φ = 0 ° to 180 °, and the average value Q [i] is obtained.
[0020]
If the sinogram is as illustrated in FIG. 2, the calculation result of the average value Q [i] is as shown in FIG. This is because the X-ray transmission data differing by 180 degrees is obtained by transmitting X-rays from the front and back of the subject W only for the channel on which the rotation axis Ra is projected. On the other hand, the value of Q gradually decreases from 1.0 as the rotation axis Ra moves away from the projection position. However, the value of Q in the channel group at both ends corresponding to the portion where the subject W does not exist is uniformly set to 1 because X-rays that always transmit only air are incident while the turntable 3 rotates 360 °. It becomes a value close to 0.0.
[0021]
Therefore, Q [i] is calculated for each channel i = 1 to n, and if a point where Q once drops from the channel on one end side reaches a local maximum value and is detected again is detected, the value of Q becomes the local maximum value. It can be recognized that the indicated channel is a channel on which the rotation axis Ra is projected. When it is necessary to obtain the rotation center axis projection position up to the sub-pixel order, interpolation calculation can be performed using the value of Q [i].
[0022]
According to the above-described method of estimating the rotation center axis projection position, when a phantom in which a tungsten wire or the like is passed through the above-described acrylic pipe as the subject W is used, a manufacturing error is caused by the estimation result of the rotation center axis projection position. Without any influence, the projection position of the rotation center axis can be accurately estimated even if an arbitrary object is used as the object. For example, the X-ray transmission of the object to actually obtain a tomographic image It is also possible to estimate the projection position of the rotation center axis using the data.
[0023]
Since the reconstruction calculation unit 52 performs the reconstruction calculation using the rotation center axis projection position estimated by the rotation center axis estimation unit 53 as described above, the tomography based on the accurate rotation center axis projection position is always performed. Since the image is reconstructed, the quality of the obtained tomographic image is always high.
[0024]
In the above-described embodiment, the average value Q of the ratio of data that differs by 180 degrees for each channel is used for calculating the degree of coincidence between X-ray transmission data that differs by 180 degrees for each channel. However, it is of course possible to use the standard deviation of each ratio or simply use the integrated value.
[0025]
Further, in the above embodiment, an example is shown in which the present invention is applied to a fan type X-ray beam irradiation type CT apparatus using a line sensor as the X-ray detector. The present invention can be applied to a so-called cone beam CT apparatus that uses a dimensional sensor and irradiates a cone-shaped X-ray beam.
[0026]
Further, in the above embodiment, the X-ray source 1 and the X-ray detector 2 are fixed, and the turntable 3 on which the subject is mounted is arranged between them, but the subject is fixed. Of course, the present invention can equally be applied to a CT apparatus of a type in which the X-ray source and the X-ray detector arranged on both sides of the sample table are integrally rotated while being placed on the sample table. is there.
[0027]
【The invention's effect】
As described above, according to the present invention, for each element of the X-ray detector, the relative rotation angle of the subject with respect to the X-ray source and the X-ray detector among the 360-degree X-ray transmission data of the subject. Is estimated as an element corresponding to the projected position of the rotation center axis of the subject by using the fact that the degree of coincidence between data that is 180 degrees different from each other is high. It is also possible to estimate the projection position of the rotation center axis from the X-ray transmission data of the above, and it is not necessary to use a dedicated phantom as in the past, and furthermore, the estimation error of the rotation center axis due to the manufacturing error of the phantom is reduced. This eliminates the possibility of occurrence, and makes it possible to constantly and stably obtain the estimated position of the rotation center axis with high accuracy, and thus, tomographic images can be stably made of high quality.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an embodiment of the present invention, in which a schematic diagram showing a mechanical configuration of a main part and a block diagram showing an electrical functional configuration are shown together.
FIG. 2 is a sinogram showing an example of X-ray fluoroscopic data used for a calculation operation of estimating a projection position of a rotation center axis performed by a rotation center axis estimation unit 53 according to the embodiment of the present invention.
FIG. 3 is a graph showing an example of a calculation result of the degree of coincidence of 180-degree difference data for each channel used in the calculation for estimating the projection position of the rotation center axis performed by the rotation center axis estimation unit 53;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 X-ray source 2 X-ray detector 3 Turntable 4 Data sampling circuit 5 Computing device 51 Preprocessing unit 52 Reconstruction computing unit 53 Rotation center axis estimating unit 6 Display W Subject

Claims (1)

An X-ray source, an X-ray detector arranged to face the X-ray source, and having a plurality of elements arranged in a direction orthogonal to an X-ray optical axis from the X-ray source; and an X-ray source and an X-ray detector. A sample stage for abutting an object arranged between the vessels, and the object on the sample stage is arranged such that the X-ray optical axis and the element are aligned with the X-ray optical axis and the array direction of the element. The sample stage is rotated so as to rotate relative to a rotation axis orthogonal to both sides, or the X-ray source and the X-ray detector are rotated, and the rotation drive unit is driven. A computed tomography apparatus comprising reconstruction calculation means for reconstructing a tomographic image obtained by slicing a subject on a plane orthogonal to the rotation axis using transmitted X-ray data detected by irradiating X-rays,
Using X-ray transmission data for 360 degrees of an arbitrary subject, a rotation center axis estimating unit utilizing the fact that the degree of coincidence between data having rotation angles differing by 180 degrees is high in an element corresponding to the rotation center axis is described. A computed tomography apparatus, comprising:

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