JP2007121094A - Computer-aided tomography apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a computer-aided tomography apparatus capable of extending a photographable region both in the direction of the rotation center axis and in the direction orthogonal to it. <P>SOLUTION: A radiation detector 2 is moved in the direction (y-axis direction) orthogonal to both the radiation optical axis R (x-axis) and rotation center axis R (z-axis) and positioned; effective widths to the y-axis direction are overlapped in respective positions, and a three-dimensional image of a subject W is built using transparency data associated with radiation taken by helical scan at each position. Thus, the effective width of the radiation detector 2 in the y-axis direction is substantially extended, and a three-dimensional image in a large region not only in the z-axis direction but also in the x-axis and y-axis directions is formed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a computer tomography apparatus that obtains a three-dimensional image of a subject using cone-beam-like radiation, and more particularly to a computer tomography apparatus that collects radiation transmission data of a subject by helical scanning.

  Conventionally, a conventional scanning method and a helical shakin method are known as an imaging method in a computed tomography apparatus using radiation such as X-rays. The conventional scanning method simply sets a pair of a radiation source and a radiation detector, which are arranged opposite to each other, and a subject simply relative to each other around a rotation center axis orthogonal to a straight line connecting the centers of the radiation source and the radiation detector. This is a method of collecting radiation transmission data while rotating it.

  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 center axis in synchronization with the rotation described above. For example, when a pair of a radiation source and a radiation detector is rotated and moved in the direction of the rotation axis, as shown in FIG. 6, they draw a spiral trajectory T about the rotation center axis R as described above. (For example, refer to Patent Document 1).

  In such a helical shakin method, a line (radiation light) connecting the radiation source S and the center of the light receiving surface of the radiation detector D is shown in FIG. On the axis (L), these pairs or the rotation center axis R of the subject are positioned, and they are swung. At that time, the CT-capable region is a range indicated by F in the figure.

Further, in the helical scan method, as shown in FIG. 8, lines connecting the radiation source S with both ends of the effective width in the direction of the rotation center axis R of the radiation detector D and the radiation source S 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 is an object W. 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 described above, the radiation in the cylindrical region within the range F in FIG. Transmission data is collected and a three-dimensional image in that region can be constructed.
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, 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 scan method, as shown in the schematic view along the rotation center axis R in FIG. 9, 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 receive light. The imaging method is arranged at a position deviating from the rotation center axis R with respect to the line L connecting the center of the surface, where Y is the deviation amount of the rotation center axis R with respect to the line L. The diameter is increased by 2Y compared to the normal scan method in which the rotation center axis R is positioned on the line L, and the radiation detector D is effective in the direction orthogonal to the rotation center axis R in comparison with the normal scan. Even if the widths are the same, the imageable area can be expanded.

  However, when such an offset scan and a 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, the imageable region in the direction of the rotation center axis is expanded, but the field of view in the direction orthogonal thereto is restricted by the effective width of the radiation detector in the same direction.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a computer tomography apparatus 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-described problems, a computer tomography apparatus according to the present invention includes a radiation source that generates cone-beam radiation and a plane (yz plane) orthogonal to a radiation optical axis (x-axis) from the radiation source. ) And a two-dimensional radiation detector having a wide area are disposed opposite to each other, and a stage is provided between the radiation source and the radiation detector for placing an object, and the radiation source and the radiation detector The pair and the stage are rotated relative to each other around a rotation center axis (z axis) orthogonal to the radiation optical axis, and at the same time, in the direction of the rotation center axis (z axis) by a certain distance per one rotation. Scan control means for collecting radiation transmission data of the subject while relatively moving the radiation source and radiation detector pair and the stage, and a three-dimensional image of the subject using the collected radiation transmission data In the computed tomography apparatus provided with the reconstruction calculation means to be constructed, the radiation detector is moved in a direction (y-axis direction) orthogonal to both the radiation optical axis (x-axis) and the rotation center axis (z-axis). Detector movement control means, the detector movement control means, at least two places where the sensitive surface of the radiation detector partially overlaps in the y-axis direction before and after the movement, and The scanning optical means is positioned so as to symmetrically cover both sides of the radiation optical axis, and the scan control means executes the scanning operation with each position of the radiation detector being positioned, The configuration calculation means is characterized by constructing a three-dimensional image of the subject using radiation transmission data collected at each position of the radiation detector.

  Here, in the present invention, the detector movement control means includes the radiation detector at even positions including a position where one end of the effective width in the y-axis direction of the detector is on the radiation optical axis. A configuration for positioning the radiation detector (claim 2) can be employed.

  Further, in the present invention, instead of the above, the detector movement control means includes the radiation detector including a position where the center of the effective width in the y-axis direction of the detector is on the radiation optical axis, A configuration (Claim 3) in which the radiation detector is positioned at an odd-numbered place may be employed.

  In the present invention, the reconstruction calculation means includes, in the radiation transmission data from the radiation detector, the data of the portions that overlap each other before and after the movement of the radiation detector, in the data collected at each position. It is possible to suitably employ a configuration in which the weight is added after being added, and the weight is made heavier toward the center of the effective width in the y-axis direction of the radiation detector at each position (Claim 4).

  The present invention moves the radiation detector in a direction (y-axis direction) orthogonal to both the rotation center axis (z-axis) and the radiation optical axis (x-axis), and performs helical scanning at each of a plurality of positions. The effective width of the radiation detector in the y-axis direction is substantially expanded to expand the imageable region in the y-axis direction.

  That is, if the radiation detector is shifted in the y-axis direction, the above-described condition shown in FIG. 7 is not satisfied, and radiation transmission data for constructing a three-dimensional image is insufficient. However, for example, as shown in FIG. 2, positioning is performed at positions A and B such that the effective widths partially overlap each other across the radiation optical axis L, and a helical scan is executed in each positioning state. The total of the collected radiation transmission data is obtained by using a radiation detector that can cover a region of C whose effective width in the y-axis direction is nearly twice that of what is actually used as a radiation detector. This is equivalent to the radiation transmission data obtained by performing a single helical scan in a state of being positioned so as to satisfy the conditions shown in FIG. Therefore, by using the radiation transmission data obtained by performing the helical scan at each position, a three-dimensional image equivalent to the case of using the detector having the effective width of the region indicated by C in FIG. Can be built.

  Here, the positioning of the radiation detector is performed by positioning the end portion of the effective width in the y-axis direction of the radiation detector on the radiation optical axis L as shown in FIG. The positions of the radiation detectors may be positioned at an even number of positions, and as illustrated in FIG. 5, that is, as in the invention according to claim 3, the center part in the y-axis direction of the radiation detector is on the radiation optical axis L. It may be positioned at an odd number of positions including the state of being positioned in the position.

  Further, regarding the handling of the radiation transmission data of the portions overlapping each other in the y-axis direction in each positioning state of the radiation detector, as in the invention according to claim 4, the center of the radiation detector at each position is larger as the center of the y-axis direction. By adding weights to each other and using them, the data can be joined smoothly, and a sense of incongruity on the three-dimensional image can be almost eliminated.

  According to the present invention, in the helical scan type computer tomography apparatus capable of extending the imageable area in the direction of the rotation center axis, the imageable area on a plane orthogonal to the rotation center axis can be expanded. Even if a radiation detector having a relatively small effective width is used, a very wide three-dimensional imageable region can be obtained.

  Further, as in the invention according to claim 4, with respect to the radiation transmission data of the portion to be overlapped in each positioning state of the radiation detector, the weight of the radiation detector at each position is given more weight, and the radiation transmission data at both positions. By adding and using, the joining of data in each positioning state of the radiation detector becomes smooth, and the sense of incongruity caused by joining can be almost eliminated in the obtained three-dimensional image.

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, and is a diagram showing a schematic front view showing a mechanical configuration and a block diagram showing a system configuration of a main part. is there. FIG. 2 is a schematic plan view of the mechanical configuration of the embodiment of the present invention.

  The X-ray generator 1 generates cone-beam X-rays from the X-ray focal point 1a, and the X-ray detector 2 is a two-dimensional X-ray detector such as a flat panel detector, and these are opposed to each other. Has been placed.

  A stage for placing a subject W is provided between the X-ray generator 1 and the X-ray detector 2. In this example, the stage is mounted with the subject W and has an X-ray optical axis L (x-axis). ) A rotary table 3 that rotates around a rotation center axis R (z axis) orthogonal to the direction, a table x direction moving mechanism 4 that moves the rotary table 3 in the X-ray optical axis L (x axis) direction, The table x direction moving mechanism 4 and the rotary table 3 are configured by a table z direction moving mechanism 5 that moves the table in the direction along the rotation center axis R (z axis).

  The X-ray detector 2 is supported by the detector moving mechanism 6, and a direction (y-axis direction) orthogonal to both the X-ray optical axis L and the rotation center axis R by driving the detector moving mechanism 6. Can be moved to. In this example, as shown in FIG. 2, the X-ray detector 2 has an effective width in the y-axis direction in the vicinity of one end portion of the X-ray detector 2 on the X-ray optical axis L. X-ray light is in a state of covering the y-axis direction region indicated by, and a position symmetrical to the y-axis direction across the X-ray optical axis L with respect to that state, that is, near the other end of the effective width in the y-axis direction. It is located on the axis L and is movable between the sensitive surfaces covering the y-axis direction region indicated by B in the figure. In the areas A and B, a few areas indicated by δ in the figure overlap each other.

  The rotary table 3 is driven and controlled by a drive signal from the rotary table drive circuit 11, and the table x direction moving mechanism 4 is driven by a drive signal from the x direction drive circuit 12, and the table z direction moving mechanism 5 is driven in the z direction. The drive is controlled by the circuit 13. The detector moving mechanism 6 is driven and controlled by a drive signal from the detector movement drive circuit 14.

  These rotary table drive circuit 11, table x-direction drive circuit 12, table z-direction drive circuit 13 and detector drive circuit 14 are placed under the control of the control unit 15. The control unit 15 is connected to an operation unit 16 for giving an instruction by an operator such as a joystick, a mouse, or a keyboard. By operating the operation unit 16, the rotary table 3, the table x-direction moving mechanism 4, The table z-direction moving mechanism 5 and the detector moving mechanism 6 can be arbitrarily driven and controlled, and at the time of CT imaging, the rotary table 3, the table z-direction moving mechanism 5 and the detection are automatically performed based on the following operations. The device moving mechanism 6 is driven and controlled. Each pixel output from the X-ray detector 2 at the time of this CT imaging is taken into the reconstruction calculation unit 17 every moment. The reconstruction calculation unit 17 constructs a three-dimensional image of the subject W by using the X-ray transmission data captured during the CT imaging operation described below, and displays it on the display 18. The control unit 15 and the reconstruction calculation unit 17 are actually configured by a computer and its peripheral devices.

  Next, the operation when performing the offset helical scan according to the above-described embodiment of the present invention will be described. FIG. 3 is a flowchart showing the operation procedure.

  First, the subject W is placed on the rotary table 3, the helical pitch Δ and the scan range in the z direction are set, and the position of the rotary table 3 in the x-axis direction in relation to the set helical pitch Δ is described above. After positioning at a position satisfying the condition of 8, a scan start command is given. Thereby, the X-ray detector 2 is positioned at a position covering the area A shown in FIG. 2, and the first helical scan is executed in this state. That is, while rotating the rotary table 3, the table z-direction moving mechanism 5 is driven to move the rotary table 3 in the z direction by the helical pitch Δ set while the rotary table 3 makes one rotation. Meanwhile, the subject W is irradiated with X-rays from the X-ray generator 1 and X-ray transmission data is taken into the reconstruction calculation unit 17. When the scan range reaches the set range, the rotary table 3 is returned to the initial position, and the X-ray detector 2 is positioned at a position covering the area indicated by B in FIG. Run a scan.

  In the reconstruction calculation unit 17, the X-ray transmission data collected by the two helical scans performed by moving the X-ray detector 2 are integrated as shown below, and then a three-dimensional image of the subject W is obtained. Build and display on display 18.

  That is, by the helical scan twice, the X-ray transmission data in the y-axis direction region indicated by C in FIG. Therefore, by performing the helical scan twice, the X-ray detector whose effective width in the y-axis direction is substantially C is arranged on the X-ray optical axis L as shown in FIG. X-ray transmission data can be obtained in the same area as in the case of performing the above, and therefore a three-dimensional image of a wide area can be constructed by using the X-ray transmission data obtained by two helical scans. .

  Here, the area A covered by the first helical scan and the area B covered by the second helical scan overlap in the area indicated by δ as described above. Regarding the handling of the data in the overlapping region δ, the data can be smoothly joined by the following manner.

That is, as shown in the graphs of FIGS. 4A and 4B, among the data obtained in the first scan and the data obtained in the second scan, the data in the region other than the overlapping region δ is multiplied by the weighting factor 1. In other words, while using the data as it is, the overlapping area data is weighted closer to 1 at the center of the effective width and closer to 0 at the end of the first and second data. Then, the first and second data at the same position are added. The example of FIG. 4A is an example in which the weighting factor to be multiplied by the overlapping region data is a straight line of 0 to 1.0 according to the position of the y-axis, and the example shown in FIG. In this example, sin ² θ is used as the weighting factor to be multiplied by the data in the region. That is, each position in the range of δ is represented by θ and regarded as 0 to π / 2, and the weighting coefficient is determined by sin ² θ using θ corresponding to each position. By adding the data of the first and second overlapping areas with such weights, the data obtained by each scan is smoothly joined, and the area represented by C as described above is obtained. Data equivalent to a helical scan using a covering detector can be obtained.

  In the above embodiment, the X-ray detector is moved to two symmetrical positions with the X-ray optical axis L interposed therebetween, and the helical scan is performed at each position. It is also possible to perform helical scanning at each position by positioning to any even number of places such as moving to both sides. Further, as shown in FIG. 5, the center of the effective width in the y-axis direction of the X-ray detector covers the area indicated by D1 in the figure located on the X-ray optical axis L and on both sides thereof. It is also possible to perform a helical scan at each position by moving to a total of three positions covering the adjacent areas D2 and D3 via the overlapping area δ, and to move to both sides of the odd number. It is also possible to perform helical scanning at each position after positioning at a location.

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 typical front view showing a mechanical structure, and the block diagram showing the system structure of the principal part. It is a schematic plan view of the mechanical configuration of the embodiment of the present invention. It is a flowchart showing the operation | movement procedure at the time of performing an offset helical scan by embodiment of this invention. It is a graph which shows the example of the weighting coefficient which multiplies the data of an overlap part among the data in each position of the X-ray detector in embodiment of this invention. It is explanatory drawing of the example of the method of positioning of the X-ray detector in other embodiment of this invention. It is the figure which expressed the locus | trajectory of the radiation source or radiation detector of a helical scan system on the coordinate axis fixed on the to-be-photographed object. It is a figure which shows the relationship of the arrangement | positioning of the radiation source of a conventional helical scan, a radiation detector, and a rotation center axis seen from the direction along a rotation center axis. It is a figure which shows the relationship between the position of a radiation source and a radiation detector, and a helical pitch required in order to extract | collect the data required in order to construct | assemble a three-dimensional image by helical scanning. It is a figure which shows the relationship of the arrangement | positioning of the radiation source in the offset scan used by a conventional scan system, a radiation detector, and a rotation center axis | shaft seen from the direction in alignment with a rotation center axis | shaft.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 X-ray generator 1a Focus 2 X-ray detector 3 Rotary table 4 Table x direction moving mechanism 5 Table z direction moving mechanism 6 Detector moving mechanism 11 Rotary table drive circuit 12 Table x direction drive circuit 13 Table z direction drive circuit 14 Detector movement drive circuit 15 Control unit 16 Operation unit 17 Reconstruction calculation unit 18 Display L X-ray optical axis R Rotation center axis W Subject

Claims (4)

A radiation source that generates cone-beam radiation and a two-dimensional radiation detector having a spread in a plane (yz plane) orthogonal to a radiation optical axis (x-axis) from the radiation source are arranged to face each other. A stage is provided between these radiation sources and radiation detectors to place the subject.
The pair of radiation source and radiation detector and the stage are rotated relative to each other about a rotation center axis (z axis) orthogonal to the radiation optical axis, and at the same time, the rotation center by a fixed distance per rotation. Scan control means for collecting radiation transmission data of the subject while relatively moving the radiation source and radiation detector pair and the stage in the direction of the axis (z-axis);
In a computed tomography apparatus provided with reconstruction calculation means for constructing a three-dimensional image of a subject using collected radiation transmission data,
Detector movement control means for moving the radiation detector in a direction (y-axis direction) orthogonal to both the radiation optical axis (x axis) and the rotation center axis (z axis) is provided. The radiation detector has at least two positions where the sensitive surface of the radiation detector partially overlaps in the y-axis direction before and after the movement, and covers both sides symmetrically across the radiation optical axis. Configured to position
The scan control means performs the scan operation in each positioned state of the radiation detector,
The computed tomography apparatus, wherein the reconstruction calculation means constructs a three-dimensional image of a subject using radiation transmission data collected at each position of the radiation detector.   The detector movement control means is configured to position the radiation detector at even positions including the position where one end of the effective width in the y-axis direction of the detector is on the radiation optical axis. The computed tomography apparatus according to claim 1.   The detector movement control means is characterized in that the radiation detector positions the radiation detector at odd places including a position where the center of the effective width in the y-axis direction of the detector is on the radiation optical axis. The computed tomography apparatus according to claim 1.   The reconstruction calculation means adds the weighted data collected at each position for the data of the radiation transmission data from the radiation detector that overlap each other before and after the movement of the radiation detector. The computed tomography apparatus according to claim 1, 2, or 3, wherein the weight is increased toward the center of the effective width in the y-axis direction.

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