JP2004061256A - Computerized tomography - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a computerized tomography for accurately estimating a projection position onto an X-ray detector at a rotary center axis and greatly contributing to the improvement of a tomographic image by placing an arbitrary object on a turntable , rotating the object, and sampling X-ray transmission data without using any exclusive phantoms. <P>SOLUTION: The computerized tomography has a rotary center axis projection position estimation means 53 that changes a rotary center axis projection position used for converting X-ray transmission data rows detected by the X-ray detector 2 in a plurality of times to parallel projection data rows, and estimates the rotary center axis projection position based on each data row being converted by using each rotary center axis projection position. The tomogram is reconfigured by the data rows being converted by using the estimated rotary center axis projection position, thus obtaining the stable, high-quality sectional image without being affected by manufacturing errors in the phantoms. <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 computer tomography apparatus using X-rays, generally, an X-ray source that generates X-rays in the form of a fan beam, and an X-ray source An X-ray detector in which a plurality of elements are arranged in the direction in which the X-ray spreads is arranged opposite to the object, and an object is mounted between the X-ray detectors and the direction of the optical axis of the X-ray and the arrangement of the elements of the X-ray detector. A configuration in which a turntable that rotates around a rotation axis orthogonal to both directions is employed.
[0003]
Then, while rotating the subject, the detection data of the X-ray intensity transmitted through the subject at every predetermined minute angle is collected from the output of each element of the X-ray detector over 360 °, and the transmission by each element at each rotation angle is obtained. Based on the X-ray detection data, a tomographic image of the subject along a plane orthogonal to the rotation axis is reconstructed.
[0004]
Here, the X-ray detector has sensitivity unevenness and distortion for each element, and transmitted X-rays incident on the X-ray detector are fan beam-like X-rays emitted from a point-like X-ray source. Since the line is transmitted through the subject, it is not possible to reconstruct a tomographic image using the element outputs of the X-ray detector as they are.
[0005]
Therefore, conventionally, sensitivity correction and distortion correction have been performed on the output of each element, and a transmission X-ray data string by each element of the X-ray detector has been converted into a parallel projection data string by a conversion called fan-para conversion. Reconstruction calculation of a tomographic image is performed using the subsequent data sequence. In this data conversion, accurate data conversion can be performed unless the projection position of the rotation center axis of the subject on the X-ray detector (hereinafter referred to as the rotation center axis projection position) is accurately known. Therefore, a high quality tomographic image cannot be obtained.
[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. From the 360 ° data train obtained by detecting the X-rays transmitted through the phantom with the X-ray detector mounted on the top and rotating it, the points where the wires were shifted to the left and right were determined. The center point of these two points is detected and estimated as the rotation center axis projection position.
[0007]
[Problems to be solved by the invention]
However, according to the above-described conventional method of estimating the rotation center projection position, the manufacturing error of the dedicated phantom directly affects the estimation result of the rotation center axis projection position, and the quality of the tomographic image depends on the degree of performance. There was a problem that was.
[0008]
The present invention has been made in view of such circumstances, and without using a dedicated phantom or the like, an arbitrary object is mounted on a turntable and rotated to collect X-ray transmission data, thereby obtaining a rotation. It is an object of the present invention to provide a computer tomography apparatus capable of accurately estimating a central axis projection position and enabling a computer to greatly contribute to high quality of a tomographic image.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, a computer tomography apparatus according to the present invention includes an X-ray source that outputs an X-ray having a fan-like spread, an X-ray source that is disposed to face the X-ray source, and an X-ray from the X-ray source. An X-ray detector having a plurality of elements arranged in a direction orthogonal to the optical axis and in the direction of spread of the X-rays; and an X-ray detector arranged between the X-ray source and the X-ray detector and mounted with a subject. A turntable rotating around a rotation center axis orthogonal to both the optical axis and the arrangement direction of the elements, and a transmitted X-ray data string detected at predetermined angles by irradiating the subject with X-rays while driving the turntable Data conversion means for converting the rotation center axis into a parallel projection data sequence using the projection position of the rotation center axis on the X-ray detector, and using the data after the conversion, the object is projected on a plane orthogonal to the rotation center axis. Equipped with reconstruction operation means for generating sliced tomographic images In the computer tomography apparatus, the rotation center axis projection position used by the data conversion means is changed over a plurality of positions, and the rotation center axis projection position is estimated based on each data sequence converted using the rotation center axis projection position. A center axis projection position estimating unit, wherein the reconstruction calculating unit is characterized by forming a tomographic image from parallel projection data converted using the rotation center axis projection position estimated by the estimating unit. 1).
[0010]
Here, as a specific and preferred method of determining the optimum rotation center axis projection position by the rotation center axis projection position estimating means in the present invention, a plurality of rotations are changed as in the invention according to claim 2. For each sinogram of a plurality of parallel projection data sequences converted using each of the center axis projection positions, the symmetry of the 180 ° opposing data was evaluated, and the parallel projection data sequence with the highest symmetry was obtained. A method of estimating the axial projection position as optimal can be suitably adopted.
[0011]
According to the present invention, an X-ray transmission data sequence of a subject detected by an X-ray detector by irradiating the subject with fan-beam-shaped X-rays is converted into a parallel projection data sequence while performing sensitivity correction and distortion correction of the X-ray detector. At the time of conversion, the rotation center axis projection position is variously changed, and the true rotation center axis projection position or the rotation center axis projection position closest to the true rotation center axis projection position is calculated from the parallel projection data sequence converted using each rotation center axis projection position. By estimating, without using a dedicated phantom, the rotation center axis projection position is accurately obtained from transmitted X-ray data using an arbitrary subject.
[0012]
Specifically, a true rotation center axis projection position is used as a rotation center axis projection position used when converting a detection data sequence of X-rays in which a fan beam-shaped X-ray has transmitted through a subject into a parallel projection data sequence. In such a case, in the data string after the conversion, for example, data that is opposed by 180 ° in the sinogram should be symmetric. Therefore, the rotation center axis projection position used when converting a transmission X-ray data string obtained by rotating an arbitrary subject by 360 ° into a parallel projection data string is variously changed, and each rotation center axis projection position is changed. By evaluating the symmetry of the 180 ° -opposed data in each sinogram of the data sequence converted using the above, the rotation center axis projected position at which the data sequence with the highest symmetry was obtained becomes the true rotation center axis. It can be estimated that it is the projection position or the rotation center axis projection position closest thereto. Then, by reconstructing the tomographic image using the data sequence converted using the rotation center axis projected position estimated in this way, the obtained tomographic image always has high quality.
[0013]
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.
[0014]
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).
[0015]
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 around a rotation center axis Ra along the line is disposed, and the subject W is mounted on the turntable 3 and is given rotation. The turntable 3 is mounted on an xyz table 31 that can move in the x, y, and z directions.
[0016]
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 takes in 360-degree X-ray transmission data of the subject W for each element of the X-ray detector 2.
[0017]
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. A data conversion unit 52 for converting the transmission data sequence into a parallel projection data sequence by performing a fan-para conversion, and a projection position of the rotation center axis Ra on the X-ray detector 2 necessary for the conversion, that is, a rotation center axis A rotation center axis projection position estimating unit 53 for estimating a projection position and a tomographic image reconstruction calculating unit 54 for generating a tomographic image using the data sequence converted by the data converting unit 52 are provided. Then, the tomographic image of the subject W generated by the tomographic image reconstruction calculation unit 54 is displayed on the display 6.
[0018]
The rotation center axis projection position estimating unit 53 estimates the rotation center axis projection position by the following method using 360 ° X-ray transmission data from each element of the X-ray detector 2 captured by the data sampling circuit 4. I do.
[0019]
That is, a plurality of rotation center axis projection positions are changed, and the captured data sequence is converted into a parallel projection data sequence using the respective rotation center axis projection positions. Then, with respect to the sinogram based on each data string after the conversion, the data which is opposed by 180 ° are compared with each other to evaluate the symmetry. Then, the rotation center axis projection position at which the data string with the highest symmetry is obtained is estimated as the true or closest rotation center axis projection position.
[0020]
FIG. 2 illustrates a sinogram. In this example, 360-degree data are arranged with the converted channel number (position in the y direction) on the horizontal axis and the rotation angle φ on the vertical axis. As a specific method of evaluating the symmetry of data that is 180 ° facing each other in such a sinogram, there are several methods, and each method will be described.
[0021]
First, in each sinogram, the data sequence of the rotation angle φ ° and the data sequence of (φ + 180) ° correspond to each other (as illustrated by an arrow in FIG. A variance value or a standard deviation value of a difference between data values (X-ray transmission amounts, in other words, brightness) of the channel data (located at symmetric positions) is obtained. Then, the rotation center axis projection position used for calculating the data sequence forming the sinogram with the smallest variance value or standard deviation value is estimated as the true projection position or the closest projection position.
[0022]
The second is that, in the sinogram, the difference between the maximum value and the minimum value of the sum of the data values of the corresponding channels in the data sequence of the rotation angle φ ° and the data sequence of (φ + 180) °, as described above. Then, the rotation center axis projection position used to calculate the data sequence forming the sinogram having the largest value is estimated as the true projection position or the closest projection position.
[0023]
Third, peak detection processing is performed on the data sequence of φ ° and the data sequence of (φ + 180) ° in each sinogram, and the peak position and the data value (X-ray transmission amount) at the peak position are obtained. Then, the peak position and its data value in the data sequence of φ ° are compared with the peak position and its data value in the data sequence of (φ + 180) ° to form a sinogram in which the mutual peak position and the data value are the best. The projection position of the rotation center axis used to calculate the data sequence is estimated as the true projection position or the closest projection position.
[0024]
In each of the above methods, φ may be an arbitrary single angle, or the value of φ may be changed from 0 to 180 °, and each evaluation result may be added or averaged to perform final processing. May be evaluated.
[0025]
The reconstruction calculation unit 54 reconstructs a tomographic image of the subject W using the parallel projection data sequence converted using the rotation center axis projection position estimated as described above.
Therefore, the tomographic image obtained by the above-described embodiment is always generated using the parallel projection data sequence converted using the accurate rotation center axis, and thus always has a high quality image.
[0026]
【The invention's effect】
As described above, according to the present invention, since the rotation center axis projection position is accurately estimated from the X-ray transmission image data of an arbitrary subject, it is not necessary to use a dedicated phantom as in the related art, There is no danger of an error in estimating the rotation center axis projection position due to manufacturing errors, and the rotation center axis projection position can always be stably and accurately estimated. It will be of high quality.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an embodiment of the present invention, and is a diagram illustrating both a schematic diagram illustrating a mechanical configuration of a main part and a block diagram illustrating an electrical functional configuration.
FIG. 2 is a diagram illustrating an example of a sinogram used for estimating a projection position of a rotation center axis performed by a rotation center axis projection position estimation unit 53 according to the embodiment of the present invention.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 X-ray source 2 X-ray detector 3 Turntable 4 Data sampling circuit 5 Computing device 51 Preprocessing unit 52 Data conversion unit 53 Rotation center axis projection position estimating unit 54 Tomographic image reconstruction calculation unit 6 Display Ra Rotation center axis W subject

Claims (2)

An X-ray source that outputs an X-ray having a fan-like spread, and a plurality of elements that are arranged to face the X-ray source, are orthogonal to the X-ray optical axis from the X-ray source, and extend in the X-ray spread direction. Are arranged between the X-ray detector and the X-ray source and the X-ray detector. The X-ray detector is mounted around a rotation center axis orthogonal to both the X-ray optical axis and the arrangement direction of the elements. Using a turntable that rotates, and a transmission X-ray data sequence detected at predetermined angles by irradiating the subject with X-rays while driving the turntable, using the projection position of the rotation center axis on the X-ray detector. Tomography data conversion means for converting the data into a parallel projection data sequence, and a reconstruction operation means for generating a tomographic image obtained by slicing a subject on a plane perpendicular to the rotation center axis using the converted data. At
Rotation center axis projection position estimating means for changing a plurality of rotation center axis projection positions used by the data conversion means and estimating a rotation center axis projection position based on each data string converted using each rotation center axis projection position. Wherein the reconstruction calculating means forms a tomographic image from the parallel projection data converted using the rotation center axis projection position estimated by the estimating means. The rotation center axis projection position estimating means evaluates the symmetry of 180 ° facing data with respect to each sinogram obtained by converting a plurality of parallel projection data strings converted using each of the plurality of rotation center axis projection positions. 2. The computer tomographic imaging apparatus according to claim 1, wherein the rotation center axis projection position at which the parallel projection data string with the highest symmetry is obtained is estimated to be optimal.

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