JP2008029771A - X-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus, which keeps the X-ray beam concentric with the X-ray detector just from the beginning of scanning, without affecting measurement time delay or X-ray tube service life. <P>SOLUTION: In the X-ray CT apparatus, the regular center position of X-ray source (tube) 1 in the slice direction when the slice-thickness center of X-ray beam 4 agrees with that of the X-ray detector 3 under various scanning conditions is stored in memory 13. The X-ray beam is shifted and adjusted so that the actual center of X-ray source 1 under the present scanning conditions can agree with the stored one. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an X-ray CT apparatus that performs X-ray beam shift correction.

The background art in the present invention will be described below.
In the X-ray CT apparatus, X-rays are emitted while a rotating unit on which an X-ray source (X-ray tube), an X-ray detector and the like are mounted rotates with respect to the subject. X-rays emitted from the X-ray tube are converted into a fan beam having a predetermined slice thickness by a collimator. The fan beam X-ray slice center and the X-ray detector slice center differ depending on scanning conditions such as tube voltage, tube current, scanner rotation speed, and tilt angle set during scanning. In addition, the X-ray tube itself generates heat (or is cooled) due to X-ray exposure (or not) for a long time, and the anode inside the X-ray tube expands and contracts to move the focal point. The slice center shifts, and as a result, the fan beam X-ray slice center, which should originally match, and the X-ray detector slice center shift. At this time, noise and pseudo images (artifacts) are generated in the reconstructed image, resulting in a deterioration in image quality. Therefore, it is necessary to correct the focus shift due to these thermal drift and mechanical displacement.

There exists patent document 1 in the example of the conventional shift correction. This is illustrated in FIG. The X-ray CT apparatus of FIG. 2 includes an X-ray source (X-ray tube) 1, a multi-channel X-ray detector 2 located opposite to the X-ray source 1, and an emission X from the X-ray source 1. A collimator that forms a fan beam X-ray 4 having a predetermined slice thickness in a direction (perpendicular to the plane of paper) perpendicular to the fan beam divergence direction (plane parallel to the plane of the paper, the divergence angle is α + β). It has a group 3 and its support mechanism 7. In addition to this, there are collecting means for collecting detection signals from the X-ray detector 2 as data, and processing means for performing preprocessing based on the collected data from the collecting means to reconstruct and obtaining a tomographic image.
JP 2005-143812 A

  The X-ray CT apparatus of FIG. 2 is characterized in that a slice direction shift detector 6 and a position driving unit 14 for detecting X-rays at the spreading end (equivalent to the spreading angle β) 5 of the fan beam X-ray 4 are provided. . The shift detector 6 detects a deviation of the fan beam X-ray in the slice direction, and controls the position of the X-ray source 1 by the position driving unit 14 so as to eliminate the deviation. Also, when the distance from the position of the X-ray fan beam 4 before the shift correction to the appropriate position after the shift correction is large, for example, when sufficient time has passed since the main measurement operation or when the gantry is tilted Prior to the main scan, shift correction is performed by performing preliminary exposure (X-ray emission with exposure to a subject blocked by a shutter).

However, in the conventional X-ray beam shift correction, the X-ray beam slice center and X-ray detector can be used immediately after the start of scanning because the X-ray beam cannot be detected unless the X-ray beam is detected by exposing the X-ray. In some cases, the slice centers do not match, and the image obtained first tends to be of poor quality.
One of the solutions to this problem is the above-described preliminary exposure. However, the preliminary exposure affects the measurement time delay and the life of the X-ray tube.

  Therefore, the present invention makes it possible to perform X-ray beam shift correction without exposing X-rays, so that the measurement time is not delayed and the life of the X-ray tube is not affected. An object of the present invention is to provide an X-ray CT apparatus in which the slice center of the X-ray beam and the slice center of the X-ray detector coincide.

The present invention relates to an X-ray source, fan beam X-ray forming means for forming X-rays generated from the X-ray source into fan beam X-rays having a predetermined first slice thickness, and an object sandwiched between X and X An X-ray detector having a predetermined second slice thickness provided at a position facing the radiation source, and processing means for acquiring a reconstructed image by collecting detection signals of the subject transmitted X-rays of the X-ray detector, With
Means for storing center position data of the X-ray source such that the center position of the slice thickness of the fan beam X-ray matches the center position of the slice thickness of the X-ray detector, as normal center position data; Means for setting the scan conditions, and control means for reading the center position data corresponding to the set scan conditions from the storage means and controlling the position of the center of the X-ray source to be the position,
An X-ray CT apparatus characterized by comprising:

  Further, according to the present invention, the stored position data of the X-ray source is a value obtained by measurement using a scanning condition as a parameter, and the control means reads out the position data read from the memory before starting scanning. An X-ray CT apparatus is disclosed in which X-ray source position data corresponding to a scanning condition is sent to the control means to control the position of the X-ray source.

  According to the X-ray CT apparatus according to the present invention, measurement can be performed in a state in which the center of the X-ray beam slice coincides with the center of the X-ray detector in the slice direction immediately after the start of scanning without performing preliminary exposure. It is possible to improve the image quality and shorten the measurement time.

FIG. 1 is an embodiment of the present invention. This X-ray CT apparatus includes an X-ray source (X-ray tube) 1, a multi-channel X-ray detector 2 having a predetermined slice thickness W ₁ provided at a position opposed to the X-ray source and an object position, and an X-ray source 1 and a collimator group 3 for forming a cone beam X-ray emitted from the fan beam X-ray 4 with a predetermined slice thickness W and a predetermined divergence angle (α + β), and its support mechanism 7, with an end divergence angle β A shift detector 6 provided on the path of the beam 5, an absolute type of the X-ray source 1 (that is, a detected value is held even when the power is turned off), a position detector 11, a position drive unit 12 for controlling the position of the X-ray source 1, and a memory 13 and a position control unit 14. The relationship of W ₁ > W is common. In addition to this, a processing unit for acquiring data of the detection signal from the X-ray detector 2 and necessary preprocessing, and further obtaining a tomographic image by reconstruction is provided. The shift detector 6 has two equal detection regions 8 and 9 as shown in an arrow A in FIG. 1, and the slide thickness W of the beam 5 is a or b with respect to the center M thereof. A difference signal between the detection signals S ₁ and S _{2 in} the detection areas 8 and 9 is output. That is,
If the difference (S ₁ −S ₂ ) = 0, the slice thickness position is the correct position,
If the difference (S ₁ −S ₂ )> 0, the slice thickness position is biased toward the detection region 8 side,
If the difference (S ₁ −S ₂ ) <0, the slice thickness position is biased toward the detection region 9 side,
I understand that there is. Therefore, the position control unit 14 captures this difference signal, and sends a control signal such that the difference signal becomes zero to the position driving unit 12 to adjust the position of the X-ray source 1 so that the X-ray source 1 is in the correct position, that is, Then, positioning is performed to the slice position of the fan beam X-ray 5 where the difference = 0.

  For such position control, data in the memory 13 is used. The memory 13 stores the target center position (tube center) of the X-ray source as data using the X-ray scanning condition as a parameter. Here, the target center position refers to the center position of the X-ray source 1 such that the slice center position of the fan beam X-ray 5 matches the slice center position of the X-ray detector 2.

The positional relationship concerning FIG. 3 is shown. In FIG. 3, 20 is an X-ray detector having a slice width R, and 21 is a fan beam X-ray (slice thickness W) viewed in the slice thickness direction. An example in which the X-ray detector 20 detects surface 20A of the slice center P ₂ and the fan-beam X-ray 21 slices the center of (the center beam C ₁ position) P ₃ matches, X-rays source 1 at this time center position P ₁ of a value stored in the memory 13.
In FIG. 3, X-rays are emitted radially from the X-ray source 1, but this is a virtual one, and is actually a result of being formed by the collimator group 3. Above.

Center position P ₁ of the X-ray source 1 when the center position P ₂ and P ₃ are matched is different by the scan conditions, to previously obtain measured in advance various scanning conditions as a parameter, in the memory 13, scan The condition is stored as a parameter.
The scan conditions that are parameters are
Tube voltage (maximum to minimum)
Tube current (maximum to minimum)
Scan time (longest to shortest)
Tilt angle (maximum to minimum)
Tube temperature (however, percentage of operating temperature range: eg 10-90%)
Refers to the focus type. Therefore, P ₁ when P ₂ and P ₃ coincide with each other as a parameter is measured in advance and stored in the memory 13.

  The pre-measurement is mainly performed at the time of manufacture and installation by the manufacturer. In addition, there are a regular inspection, a time immediately before the actual measurement of the subject, a few times before the daily scan, and the like. However, the above combinations are very large numbers, and important scan conditions and scan conditions that have a large effect on accuracy are determined in advance and measured relatively frequently. Do.

The shift detector 6, the position detector 11, the position control unit 14, and the position driving unit 12 are used for measuring the center position P ₁ . The processing flow is shown in FIG.
In the flow 100, a sequence mode for storing (registering) the X-ray tube slice direction position is set. This depends on the operator's specification, but there is an automatic setting. In the flow 101 for each sequence, scan conditions (parameters) related to the above combinations are set one after another. For each scan conditions, measurement start (flow 102) the difference signal _(S 1 -S ₂₎ of the shift detector 6 performs, equal to the position control section 14, X-ray source through the position drive unit 12 1 Is controlled (flows 103 and 104). The center position of the X-ray source when they are not equal is detected by the position detector 11, and this is stored in the memory 13 as the normal center position of the X-ray source 1 under the current scanning conditions (flow 105). This is performed for all combinations of scan conditions. These repetitions are programmed as a sequence and are automatic measurements.

FIG. 5 is a processing flow of positioning to the position P ₁ using the data in the memory 13. This is performed immediately before the actual measurement of the subject. The actual scanning conditions are determined, whereby the center position P ₁ by the scan condition is because it determined as the center position of the normal.

The scan condition is set in the flow 110 of FIG. It reads the normal center position data _{P 1} from the memory 13 for the scanning conditions in the flow 111. In the flow 112, the current slice direction position P _i is detected by the detector 11, and it is checked whether or not the current position P _i matches the data P _1, and if not, the X-ray source is moved so as to match. (Flows 113 and 114). At this coincident position, actual measurement is performed on the subject.

Here, the flows 113 and 114 are performed by the position control unit 14 and the position driving unit 12. At this time, the control unit 14 and the drive unit 12 can be realized by applying the feedback control to zero the difference between the detection value P _i of the position detector and read data P ₁ in the memory 13.
The multi-channel X-ray detector can be used in both the single slice case and the multi-slice case. Further, although the normal position of the X-ray source is set as the center position, there is a case where a position other than the center position where the relative position to the center position is determined is set as the normal position.

It is an embodiment figure of the X-ray CT apparatus of the present invention. It is a figure which shows the conventional X-ray CT apparatus. It is explanatory drawing of the slice position of this invention. It is a figure which shows the processing flow of acquisition to the memory | storage of the normal position of the X-ray source 1 by making measurement scan conditions into a parameter of this invention. It is a control flowchart which controls the position of the X-ray source 1 to the normal position preserve | saved at memory.

Explanation of symbols

1 X-ray source (X-ray tube)
2 Multi-channel X-ray detector 3 Collimator group 6 Shift detector 11 Absolute type position detector 12 Drive controller 13 Memory 14 Position controller

Claims (2)

An X-ray source, fan beam X-ray forming means for forming X-rays generated from the X-ray source into fan beam X-rays having a predetermined first slice thickness, and facing the X-ray source across the subject An X-ray detector having a predetermined second slice thickness, and a processing means for acquiring a reconstructed image by collecting a detection signal of the subject transmission X-ray of the X-ray detector,
Means for storing center position data of the X-ray source such that the center position of the slice thickness of the fan beam X-ray matches the center position of the slice thickness of the X-ray detector, as normal center position data; Means for setting the scan conditions, and control means for reading the center position data corresponding to the set scan conditions from the storage means and controlling the position of the center of the X-ray source to be the position,
An X-ray CT apparatus comprising: The stored position data of the X-ray source is a value obtained by measurement using a scan condition as a parameter, and the control means corresponds to the scan condition read from the memory before the start of scanning at the time of position control. 2. The X-ray CT apparatus according to claim 1, wherein the X-ray source position data is sent to the control means to control the position of the X-ray source.

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