JP2007114164A - Computer tomography method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a computer tomography method for expanding a photographable area in both of the rotation center axis direction and the direction crossing it at right angles. <P>SOLUTION: In CT photographing based on a helical scan method, helical scanning is carried out twice in the position deviated in the rotation center axis R direction by a half of a helical feed pitch Δ satisfying a condition for photographing an object W by helical scanning while the rotation center axis R is positioned in a position deviated from a line L connecting the center of a light receipt face of a radiation detector 2 and a radiation ray source 1a together in the visual field of the radiation detector 2. In this way, radioparency data necessary for constructing a three-dimensional image of the object W are acquired. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a computer tomography method for obtaining a three-dimensional image of a subject using cone-beam-like radiation, and more particularly to a computer tomography method for collecting radiation transmission data of a subject by helical scanning.

  Conventionally, as a tomography method, a conventional scan method and a helical scan method are known. The conventional scanning method is a method in which a pair of a radiation source and a radiation detector, which are arranged opposite to each other, and a subject are simply relatively moved around a rotation axis orthogonal to a straight line connecting the center of the radiation source and the radiation detector. This is a method of collecting radiation transmission data while rotating.

  On the other hand, the helical scan method is a method of collecting radiation transmission data while relatively moving a pair of a radiation source and a radiation detector and a subject in a direction along the rotation axis in synchronization with the above rotation. When a pair of a radiation source and a radiation detector is rotated and moved in the direction of the rotation axis, they are given such names because they draw a spiral trajectory (see, for example, Patent Document 1).

  In such a helical scan method, the radiation source S and the center of the light receiving surface of the radiation detector D are connected as shown in a schematic arrangement view along the direction of the rotation axis central axis R in FIG. On the line L, these pairs or the rotation center axis R of the subject are positioned, and then they are slackened. At that time, the CT-capable region is a range indicated by C in the figure, and the subject must be arranged within this range.

In the helical scan method, as shown in FIG. 6, lines connecting the radiation source S and the effective width both ends of the radiation detector D in the direction of the rotation center axis R are E1 and E2, and the relative rotation is one rotation. When the position of the radiation source S before the rotation is A and the position after one rotation is B, the point P where the upper line E1 before one rotation intersects the line E2 after one rotation intersects the subject W. Must be positioned on the radiation source S side, and the helical pitch Δ is set so as to satisfy this condition. By satisfying this condition and the positional relationship between the rotation center axis R, the radiation source S, and the radiation detector D, radiation transmission data necessary for constructing a three-dimensional image of the subject W can be obtained. .
JP 2004-89720 A

  By the way, according to the helical scan method as described above, the imageable area in the direction of the central axis of rotation is greatly expanded as compared with the size of the radiation detector. It is very advantageous to obtain an image.

  However, in the direction orthogonal to the rotation center axis, there is a problem that the imageable region is limited by the effective width of the radiation detector. Such expansion of the imageable area in the direction orthogonal to the rotation center axis is known as an offset scan method in conventional scanning.

  In the offset scanning method, as shown in the schematic view along the rotation center axis R in FIG. 7, the rotation center axis R is placed in the field of view of the radiation detector D, but the radiation source S and the radiation detector D are arranged. An imaging method in which the line L connecting the center of the light receiving surface is arranged at a position deviating from the rotation center axis R, where Y is the amount of deviation of the rotation center axis R from the line L. Is larger by 2Y than the normal scan method in which the rotation center axis R is positioned on the line L. Compared with the normal scan, the diameter of the radiation detector D in the direction orthogonal to the rotation center axis R is Even if the effective width is the same, the imageable area can be expanded.

  However, when such an offset scan and helical scan are combined, there is not enough radiation transmission data for constructing a three-dimensional image of the subject, so such an imaging method has not been realized. As described above, although the imageable region in the direction of the rotation center axis is expanded, the field of view in the direction orthogonal to the region is limited by the effective width of the radiation detector in the same direction.

  The present invention has been made in view of such a situation, and an object of the present invention is to provide a computer tomography method capable of extending the imageable region both in the direction of the rotation center axis and in the direction orthogonal thereto.

  In order to solve the above problems, a computer tomography method of the present invention includes a radiation source that generates cone-beam radiation, a two-dimensional radiation detector, and a pair of the radiation source and the radiation detector. The subject is rotated relative to the rotation axis orthogonal to the radiation optical axis, and in synchronization with the rotation, the subject is moved relative to the rotation axis direction based on the required helical feed pitch. In a computed tomography method for obtaining a three-dimensional image of a subject by reconstructing the acquired data and performing CT imaging by a helical scan method for acquiring the radiation transmission data of the object, the rotation axis is set within the field of view of the radiation detector. And the helical feed pitch in a state of being deviated from a straight line connecting the center of the light receiving surface of the radiation detector and the radiation source. Characterized by performing two helical scans at a position shifted in the direction of the rotation axis by ½, and reconstructing the radiation transmission data of the subject collected in each helical scan to obtain a three-dimensional image of the subject. .

  In the present invention, the offset scan is executed by the helical shakin method. However, the lack of radiation transmission data due to the fact that the helical pitch is performed in a different arrangement from the state of FIG. 5 is shown in FIG. Covering is performed by performing two helical scans at a position shifted by ½ of the indicated helical pitch Δ.

  That is, when performing the helical scan based on the helical pitch Δ necessary for obtaining the imageable region of the subject W shown in FIG. 6, when the offset scan shown in FIG. Radiation transmission data is insufficient by the amount deviating from the line connecting the center of the vessel D and the radiation source S. This is because, in the offset scan shown in FIG. 7, the region that is within the imageable region C in the state of FIG. 7 but is outside the field of view of the radiation detector D is about 180 ° after the relative rotation in the conventional scan method. Although the radiation transmission data can be collected within the field of view of the radiation detector D, in the helical scan method, after the relative rotation of 180 ° from the state shown in FIG. 7 and the radiation detector D move relative to the subject in the direction of the rotation center axis R, so that the position is within the imageable region C and outside the field of view of the radiation detector D at the planar position shown in FIG. This is because the area to be moved already moves in the direction of the rotation center axis R and deviates from the field of view in the same direction.

  Therefore, in the present invention, two helical scans are executed at a position shifted by 1/2 of the helical pitch in the direction of the rotation center axis R. As a result, the region deviating from the field of view of the radiation detector in the first helical scan enters the field of view of the radiation detector in the second helical scan, and the radiation transmission data collected in these two helical scans By summing up, almost all radiation transmission data in the imageable region C shown in FIG. 7 can be obtained, and a three-dimensional image in the region C can be constructed in all planes in the direction of the rotation center axis. It becomes.

  According to the present invention, the helical scan method can enlarge the imageable area in the direction of the rotation center axis with respect to the field of view of the radiation detector, and also in the direction perpendicular to the center of rotation of the radiation detector. Since an offset scan that can expand the imageable area is performed, a 3D image of a wide area can be obtained using a radiation detector with a relatively small field of view, and a 3D image of a wide area can be obtained with an inexpensive configuration. An image can be constructed.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a configuration diagram of an embodiment in which the present invention is applied to an X-ray CT apparatus. FIG. 1A is a schematic diagram illustrating a mechanical configuration and a block diagram illustrating a system configuration of a main part. FIG. Further, (B) shows a schematic plan view of the vicinity of the rotary table 3.

  In this example, the X-ray generator 1 generates cone beam X-rays from the focal point 1a, and the X-ray detector 2 is a two-dimensional X-ray detector such as a flat panel detector, which are opposed to each other. Are arranged as follows.

  Between the X-ray generator 1 and the X-ray detector 2, a turntable 3 on which a subject W is mounted is disposed. The turntable 3 rotates around a rotation center axis R (z-axis direction) orthogonal to the x-axis direction along the X-ray optical axis L from the X-ray generator 1. The turntable 3 is placed on an xy table 4 that moves in the x and y axis directions orthogonal to each other on a plane orthogonal to the rotation center axis R. Further, the xy table 4 and the rotary table 3 thereon are mounted on a z-direction drive mechanism 5 that moves in the z-axis direction.

  The rotary table 3 is driven and controlled by a drive signal from the rotary table drive circuit 11, the xy table 4 is driven and controlled by a drive signal from the xy table drive circuit 12, and the x-direction drive mechanism 5 is driven by an x-direction drive mechanism. Drive control is performed by a drive signal from the circuit 13.

  The rotary table drive circuit 11, the xy table drive circuit 12 and the x-direction drive mechanism drive circuit 13 are placed under the control of the control unit 14. The control unit 14 is connected to an operation unit 15 for giving an instruction by an operator such as a joystick, a mouse, or a keyboard. By operating the operation unit 15, the rotary table 3, the xy table 4, and the z-direction drive are performed. The mechanism 5 can be arbitrarily driven, and at the time of CT imaging, the rotary table 3 and the z-direction drive mechanism 5 are automatically controlled based on the following operations. Each pixel output from the X-ray detector 2 at the time of this CT imaging is taken into the reconstruction calculation unit 16 every moment. The reconstruction calculation unit 16 constructs a three-dimensional image of the subject W using the X-ray transmission data of the subject W captured during the CT imaging operation described below, and displays it on the display unit 17. In addition, the control part 14 and the reconstruction calculation part 16 are actually comprised by the computer and its peripheral device.

  Next, the operation in the case of performing CT imaging by the offset helical scan method according to the above-described embodiment of the present invention will be described. FIG. 2 shows a flowchart of the operation procedure.

  As shown in FIG. 2, first, the subject W is mounted on the rotary table 3, the xy table 4 is operated, and the position of the rotation center axis R is changed to the X-ray generator 1 as shown in FIG. Is positioned at a position deviating from a line (X-ray optical axis) L connecting the focal point 1a and the center of the X-ray detector 2. Next, the z-direction drive mechanism 5 is operated to position the rotary table 3 at the initial scan position in the z-axis direction. Also, the scan end position in the same direction is set. Also, a helical pitch Δ is set, and this helical pitch Δ satisfies the above-described condition of FIG. Further, after setting the direction of the helical scan, for example, moving from the bottom to the top, an offset helical scan start command is given.

  By giving this command, the control unit 14 drives the rotary table 3 and the z-direction drive mechanism 4 in synchronization with each other while irradiating the subject W on the rotary table 3 with the X-rays from the X-ray generator 1, and rotates. While the table 3 is rotated once, the z-direction drive mechanism 4 is raised by Δ, and during that time, the X-ray transmission data of the subject W is taken into the reconstruction calculation unit 16 for each specified minute angle. That is, the helical scan is performed based on the helical pitch Δ.

  When the position of the rotary table 3 in the z direction reaches the end position, the scanning is stopped, the rotary table 3 is automatically lowered to a position shifted by Δ / 2 from the initial position in the z-axis direction, and the above is again performed from that position. A similar second helical scan is performed. After completing the scan for the same number of rotations and movement distance as the first time, the reconstruction calculation unit 16 constructs a three-dimensional image of the subject W using the X-ray transmission data obtained by these two scans.

  Considering a coordinate system that moves with the subject W in the above imaging operation, the trajectory of the X-ray focal point 1a of the X-ray generator 1 is as shown in FIG. 3 in the first scan S1 and the second scan S2. Then, at the same position in the z-axis direction, they are moved 180 ° apart from each other. Considering an arbitrary plane in the z-axis direction indicated by Q in FIG. 3, as shown in FIG. 4 (A), the plane Q in the first shack is provided with an offset. A region Qa where transmission data cannot be obtained appears, but in the second scan, the X-ray focal point 1a passes through the plane Q at a rotation angle α + 180 °, and as shown in FIG. Data of the area Qa where data could not be obtained can be obtained.

  Therefore, the X-ray transmission data collected as described above covers the entire area of the subject W, and by using these data, a three-dimensional image of the subject W can be constructed, and the X-ray detector 2 Compared to the sensitive surface, a three-dimensional image of a wider area can be obtained both in the direction of the rotation center axis and in the direction orthogonal thereto.

  In the above embodiment, an example in which the subject W is rotated and moved to perform the helical scan is shown. However, the pair of the X-ray generator 1 and the X-ray detector 2 may be rotated and moved. Further, a rotation is applied to the pair of the X-ray generator 1 and the X-ray detector 2 to move the subject W in the z direction, or the subject W is rotated, and the X-ray generator 1 and the X-ray detector 2 are moved. Even if it is moved in the z direction, the same effect as the above-described example can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS It is a block diagram of embodiment which applied this invention to the X-ray CT apparatus, and is the figure which writes together and shows the schematic diagram showing a mechanical structure, and the block diagram showing the system configuration | structure of the principal part. It is a flowchart showing the operation | movement procedure in the case of performing CT imaging by an offset helical scan system by embodiment of this invention. It is explanatory drawing of the locus | trajectory of the X-ray focus 1a on the coordinate system which moves with the to-be-photographed object, when offset helical scanning is performed by embodiment of this invention. It is explanatory drawing (A) of the data non-collection area | region of the 1st scan in embodiment of this invention, and explanatory drawing of the area | region collecting data by the 2nd scan. It is explanatory drawing of arrangement | positioning of the conventional helical scan system, and is the typical arrangement | positioning figure seen along the direction of the rotating shaft central axis R. FIG. It is explanatory drawing of the conditions which set the helical pitch in the case of image | photographing by a helical scan system. It is explanatory drawing of arrangement | positioning of offset scanning.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 X-ray generator 1a X-ray focus 2 X-ray detector 3 Rotary table 4 xy table 5 z direction drive mechanism 11 Rotation table drive circuit 12 xy table drive circuit 13 z direction drive mechanism drive circuit 14 Control part 15 Operation part 16 Re Configuration calculation unit 17 Display R Rotation center axis W Subject

Claims (1)

A radiation source that generates cone-beam radiation and a two-dimensional radiation detector are arranged opposite to each other, and the radiation source, the pair of radiation detectors, and the subject are relative to each other about a rotation axis orthogonal to the radiation optical axis. In synchronization with the rotation, the CT scan is performed by the helical scan method in which the radiation transmission data of the subject is collected by moving relative to the rotation axis direction based on the required helical feed pitch. In a computer tomography method for reconstructing collected data to obtain a three-dimensional image of a subject,
The rotational axis of the helical feed pitch is arranged in a state where the rotation axis is located within the field of view of the radiation detector and deviated from the straight line connecting the center of the light receiving surface of the radiation detector and the radiation source. Two helical scans are performed at a position shifted in the direction of the rotation axis by ½, and the radiation transmission data of the subject collected by each helical scan is reconstructed to obtain a three-dimensional image of the subject. Computed tomography method.

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