JP3219106B2 - CT scanner - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CT scanner which reconstructs an image by irradiating a subject with radiation such as X-rays and subjecting the collected data to computer processing, and more particularly to a helical scan type CT scanner. Regarding improvement.

[0002]

2. Description of the Related Art Conventionally, a helical scan type CT has been used.
A scanner is known (for example, see Japanese Patent Application Laid-Open No. 3-103).
229). In this method, an X-ray tube is rotated around the subject while moving the subject in one axis direction, and a helical scan is performed on the subject to obtain spiral projection data, thereby reconstructing an image. Things.

In this case, the position at which the X-ray beam passes through the subject is always moving in the axial direction (because the subject is moving), and the projection data in each direction is the same slice of the subject. Since the surface is not scanned, it is known that the image quality is degraded when the image is reconstructed using these.

In order to eliminate this deterioration in image quality, it has been considered that the X-ray beam is swung in the above-mentioned moving direction so as to pass through the same slice plane during one rotation.

[0005]

However, when the X-ray beam is swung in the moving direction, the X-ray beam can pass through the same slice plane within one rotation. However, there is still a problem that it is still insufficient and an artifact is generated.

That is, since the X-ray beam crosses the slice plane obliquely in order to allow the X-ray beam to pass through the same slice plane, it is possible to pass parts other than the slice plane by making the oblique. In other words, if there is a portion near the slice plane where the X-ray absorptivity greatly changes in the above moving direction, the effect appears and an artifact appears in the reconstructed image, thereby deteriorating the image.
For example, when the slice plane size of the subject is large,
When the X-ray absorptivity greatly changes at a site having a complicated structure in the body axis direction such as the ribs or the fundus, the effect of tilting the X-ray beam is remarkable, and this problem occurs.

SUMMARY OF THE INVENTION In view of the above, the present invention eliminates as much as possible the effect of tilting a beam in a helical scan system when the beam is swung so as to pass through the same slice plane within one rotation. An object of the present invention is to provide a CT scanner improved so as to reduce artifacts of a reconstructed image.

[0008]

In order to achieve the above object, in a CT scanner according to the present invention, means for moving a subject in one axial direction, and irradiating the moving subject with a radiation beam. Means for obtaining projection data in each direction around the axis by rotating the irradiation direction around the axis, and slicing as the object moves during rotation in the beam irradiation direction. In accordance with the movement of the subject during each of the rotations described above, each rotation of the rotation is performed so that the beam being rotated always passes through the same slice plane within one rotation despite the movement of the plane. Means for oscillating the beam within a plane including the axis within a predetermined angle range, and the projection data of each direction in each rotation within one rotation which is 180 ° opposite to each other, having a large oscillating angle. Weight that gets smaller Means for adding while weighting by a coefficient are provided, and means for reconstructing an image using projection data in each rotation obtained by the addition. In the helical scan method, the radiation beam scans the same slice plane within one rotation. When the beam is swung so as to pass, the projection data in the 180 ° opposite direction of the projection data in each direction in each rotation is added while weighting with a weighting factor that becomes smaller as the swing angle becomes larger. It is characterized in that an image is reconstructed using the projection data in each one rotation, and the weighted average processing of such data allows the beam to pass through the same slice plane without tilting in one rotation. The effect of tilting the beam is reduced because projection data is obtained as if it had been obtained in the reconstructed image, and artifacts in the reconstructed image are greatly reduced.

[0009]

An embodiment of the present invention will be described below in detail with reference to the drawings. In FIG. 1, an X-ray tube 11
X-ray beam passes through the variable collimator 12
The X-ray beam transmitted through the subject 21 enters the detector 13, and the data collection device 14 collects projection data.

The X-ray tube 11 is supplied with a high voltage from a high voltage generator 15. The high voltage generator 15 is controlled by an X-ray controller 16, which determines the timing of the X-ray beam emitted from the X-ray tube 11. The variable collimator 12 oscillates the X-ray beam in a direction perpendicular to the paper surface (Z direction), and is controlled by a collimator control device 17. The subject 21 is lying on a bed table 22, and the table 22 is moved in the Z direction by a table controller 23.

Assuming that the X direction is the right direction in the plane of the paper, the Y direction is the upward direction in the plane of the paper, and the body axis of the subject 21 is the Z axis, the X-ray tube 11 is rotated around the Z axis. The variable collimator 12 and the detector 13 are integrally rotated as shown by an arrow 19 under the control of the rotation control device 18. By such a rotational movement of the X-ray beam, XY perpendicular to the Z axis is obtained.
Projection data in each direction in the plane is obtained.

The projection data is sent to a weighting and adding device 25 via a data bus 24 and weighted and added, and then sent to an image reconstructing device 26 to reconstruct an image. The image is displayed by an image display device 28. You. X-ray controller 1
6, a collimator control device 17, a rotation control device 18, a table control device 23, and the like are connected to a control computer 27 via a data bus 24 to control the rotation angle signal of the X-ray tube 11 and the like and the movement of the bed table 22. The projection data is collected in association with the position signal at the time, and the control computer 27 controls the movement of the bed table 22 and the swing angle of the variable collimator 12 in synchronization with the rotational movement of the X-ray tube 11 or the like. , X-ray irradiation timing control, and the like.

FIG. 2 is a diagram for explaining the swing of the X-ray beam, and is a schematic diagram viewed from the right in FIG. The bed table 22 is moved in the Z direction as indicated by the arrow 42, and the subject 21 is moved in the same direction. The variable collimator 12 moves the slit position in the left-right direction in FIG. 2 so that the X-ray beam from the X-ray tube 11
1 in the movement direction (Z direction). This swing angle θ
Is controlled by the collimator controller 17 and is changed in synchronization with the rotational movement of the X-ray beam.

That is, assuming that the X-ray beam makes one rotation while the subject 21 moves by the distance d in the direction of the arrow 42, the position of the X-ray focal point as shown in FIG. Has moved from point 31 to point 33 by distance d in the direction of arrow 43. At the beginning of one rotation (rotation angle 0 °), the focal position is at point 31 and the rotation angle is 180 °
Assuming that a point 32 is reached at 360 ° and a point 33 is reached at 360 °, the swing angle θ of the X-ray beam during this one rotation (360 °)
Is shifted little by little so that the X-ray beam always passes through the same slice plane 41.

The swing angle θ is defined as the distance r from the center of rotation (Z axis) of the focal position, and the rotational angle of the focal point is φ (φ
= 0 ° to 360 °), θ = tan ^-1 [d {(φ-180 °) / 360 °} / r] (°) ≒ d {(φ-180 °) / 360 °} · ｛360 ° / (r · 2π)｝ (°) ≒ (d / r) · {(φ−180 °) / 2π} (°).

Therefore, if the table 22 is fed by d = 10 mm in one rotation (φ = 0 ° to 360 °) and r = 570 mm (these are typical values), the swing angle θ becomes the rotation angle φ. -0.5 ° to +0.5 as shown in FIG.
In the range of °.

The projection data in the 180 ° opposite direction are originally added by the X-ray beams passing through the same path on the same slice plane, and therefore are added to each other. In practice, however, the X-ray beam fluctuates as described above. Since the light passes through the slice plane while being inclined by the angle θ (see FIG. 3), it is added after being weighted by the weighting addition circuit 25 with the weighting coefficient k (φ) corresponding to the rotation angle φ. That is, when the swing angle θ is large, the contribution is small, and when the swing angle θ is small (θ is close to 0 °) and the X-ray beam passes through the slice plane 41 in parallel, the contribution is large. I do. Thus, when the swing angle θ is large and crosses the slice surface 41 obliquely, the X-ray beam passes through a portion other than the slice surface 41, and the transmitted X-ray intensity changes abruptly. Influence can be reduced.

The weight coefficient k (φ) is 0 ≦ k (φ) ≦
Take the value of 1, for example, k (φ) = | θ (φ + 180 °) | / {| θ (φ) | + | θ (φ + 180
°) |}. Then, the projection data P (φ) obtained at the rotation angle φ and the direction 180 ° opposite thereto (φ + 1)
80 () and projection data P (φ + 180 °) are weighted and added as shown by the following equation: k (φ) · P (φ) + {1-k (φ)} · P (φ + 180 °) (In this case φ
= 0 ° to 180 °).

In this case, the relationship between the movement distance of the subject 21 with respect to the rotation of the X-ray beam, the swing angle θ, and the weighting coefficient k (φ) is as shown by A, B, and C (solid lines) in FIG. , 5m
The images of the slice planes at 10 mm intervals located at m, 15 mm, 25 mm,... are reconstructed.

The weight coefficient k (φ) is not limited to a linear one as shown by the above equation and the solid line in FIG.
Anything may be used as long as the degree of contribution of the large swing angle θ is reduced, and it may be represented by a smooth curve as shown by a dotted line in FIG. 5C.

In the above description, as shown in FIG. 5A, the subject 21 is moving at a constant speed. However, the moving speed may be slowed (stopped) at the slice plane position where an image is obtained. To reconstruct an image of a slice plane at 10 mm intervals located at 5 mm, 15 mm, 25 mm,..., The X-ray beam is rotated at a constant angular velocity as shown in FIG. The movement of the bed table 22 is stopped at each slice position at 10 mm intervals corresponding to when φ is around 180 °. At this time, the rotation angle φ
Since the X-ray focal position and the slice position coincide in the Z direction in a wide range around 180 °, the swing angle θ may be 0 ° during this stop. Therefore, the swing angle θ (φ) is as shown in FIG. The weighting coefficient k (φ) is set as shown in FIG. 6C in accordance with the swing angle θ (φ).

As shown in FIG. 6, the projection data obtained by the X-ray beam oblique to the slice plane is reduced, and the weighted addition of the projection data in the 180 ° opposite direction is performed. Since the influence of the projection data obtained by the changed X-ray beam is reduced, artifacts in the reconstructed image can be further reduced.

In the above embodiment, the detector 13 is connected to the X
Although the so-called third-generation X-ray CT apparatus that rotates integrally with the tube 11 has been described, a so-called fourth-generation X-ray apparatus in which a detector is provided over 360 ° and does not need to rotate as a detector is required. Of course, the present invention can be applied to a line CT apparatus, a CT apparatus that scans an electron beam at high speed, or a CT scanner that uses another radiation source without using an X-ray beam as a radiation beam.

[0024]

According to the CT scanner of the present invention, in the helical scan method, data is collected by oscillating the radiation beam so as to pass through the same slice plane within one rotation, and collects data in each direction within each rotation. One of the projection data
80 ° opposite directions are added together. At this time, the addition is performed while weighting with a weighting factor that becomes smaller as the swing angle becomes larger, so that the effect of tilting the beam due to the swing is substantially eliminated. Thus, artifacts of the reconstructed image can be significantly reduced. Therefore, it is possible to improve image quality in high-speed continuous scanning.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating the swing of an X-ray beam in the embodiment.

FIG. 3 is a schematic diagram illustrating an X-ray focal position and a swing angle of the X-ray beam in the embodiment.

FIG. 4 is a view for explaining the relationship between the rotation angle and the swing angle of the X-ray beam in the embodiment.

FIG. 5 is a graph showing a relationship among a moving distance, a swing angle, and a weight coefficient with respect to the rotation of the X-ray beam in the embodiment.

FIG. 6 is a graph showing another example of the relationship among the moving distance, the swing angle, and the weight coefficient with respect to the rotation of the X-ray beam.

[Explanation of symbols]

 Reference Signs List 11 X-ray tube 12 Variable collimator 13 Detector 14 Data collection device 15 High voltage generator 16 X-ray controller 17 Collimator controller 18 Rotation controller 21 Subject 22 Bed table 23 Table controller 24 Data bus 25 Weighting and adding device 26 Image Reconstruction Device 27 Control Computer 28 Image Display Device 31, X-ray Focus Position 41 Slice Surface

Claims (1)

(57) [Claims] A means for moving the subject in one axial direction;
Means for irradiating the moving subject with a radiation beam and rotating the irradiation direction around the axis to obtain projection data in each direction around the axis; and Even though the slice plane moves as the subject moves, the rotating beam always passes through the same slice plane within one rotation,
Means for oscillating the beam within a predetermined angle range within a plane including the axis for each rotation of the rotational movement following the movement of the subject during each rotation; Means for adding projection data in the opposite direction of 180 ° among the projection data in each direction while weighting the projection data with a weighting coefficient that becomes smaller as the swing angle becomes larger.
Means for reconstructing an image using the projection data in the rotation.