JP2004105237A - Method for data correction and x-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct data by distinguishing abnormal discharge (spits) occurring in an X-ray tube from incidence disturbance (ref. obstruction) of an X-ray to the reference channel of a detector. <P>SOLUTION: The method for data correction monitors reference channel data Ref_j and tube voltage kVj by an X-ray CT apparatus. When lowering in the reference channel data Refj and lowering in the tube voltage kVj are detected and when lowering in the reference channel data Refj is detected and its former time is the state of abnormal state, the method determines the cases to be abnormal discharge. When lowering in tube current is not detected in spite of detecting lowering in the channel data and the former time is not the state of abnormal, the method determines the case to be reference channel obstruction (P1 and P2). In the case of determining the case as reference channel obstruction, the method never performs abnormal discharge correction (P4). Thus, the method avoids wrongly performing unnecessary abnormal discharge correction in the case of the incidence obstruction of the X-ray on a reference channel. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a data correction method and an X-ray computed tomography (CT) apparatus, and more particularly, to abnormal discharges (spitz) occurring in an X-ray tube and impingement of X-rays on a reference channel of a detector (ref. The present invention relates to a data correction method and an X-ray CT apparatus capable of correcting data while distinguishing the data from an X-ray CT.
[0002]
[Prior art]
Japanese Patent Application Laid-Open No. 2002-058668 describes an example of a reference correction method performed using data obtained on a reference channel.
[0003]
Japanese Patent Application Laid-Open No. H11-283789 describes an example of a method for coping with abnormal discharge occurring in an X-ray tube.
[0004]
[Problems to be solved by the invention]
The channel at both ends or one end of the detector is used as a reference channel. If the position of the subject is incorrect or the size of the subject is very large, X-rays incident on the reference channel may vary depending on the view angle. There is a case where interference is caused by the subject (this is called X-ray incidence interference), and the reference channel data becomes smaller than a normal value. In this case, it is necessary to consider correction for X-ray incident interference for reference channel data, but it is not necessary to consider correction for X-ray incident interference for data of other channels.
[0005]
On the other hand, if an abnormal discharge occurs in the X-ray tube, both the reference channel data and the data of the other channels become smaller than normal values. It needs to be considered.
[0006]
However, conventionally, there has been a problem that a process for distinguishing between X-ray incident interference and anisotropic discharge has not been performed.
Therefore, an object of the present invention is to provide a data correction method and an X-ray CT apparatus that can correct data by distinguishing between abnormal discharge and disturbance of X-ray incidence on a reference channel.
[0007]
[Means for Solving the Problems]
According to a first aspect, the present invention monitors reference channel data and tube voltage with an X-ray CT apparatus, detects a decrease in reference channel data, but does not detect a decrease in tube current and has not been in an abnormal discharge state last time. In such a case, there is provided a data correction method characterized in that abnormal discharge correction is not performed in the case of the above.
In the data correction method according to the first aspect, even if the first condition that “reference channel data decreases” is satisfied, the second condition that “tube voltage decreases” or “the last time was in an abnormal discharge state” is satisfied. If the above condition is not satisfied, abnormal discharge correction is not performed. For this reason, it is possible to prevent unnecessary abnormal discharge correction from being performed when X-rays enter the reference channel.
The reason that the second condition includes "the previous time was in the abnormal discharge state" is that even if the abnormal discharge stops and the tube voltage is restored, the restoration of the X-ray dose may be delayed. is there.
[0008]
According to a second aspect, the present invention provides a data correction method having the above-described configuration, in which a decrease in reference channel data and a decrease in tube voltage are detected, and a decrease in reference channel data is detected and the previous time was in an abnormal discharge state. In this case, when the drop in the tube voltage is small, the data of the corresponding view is used, and when the drop in the tube voltage is large, the data of the corresponding view is corrected by the data of the previous and subsequent views. Provide a method.
In the data correction method according to the second aspect, when the drop in the tube voltage is small during abnormal discharge, the data of the corresponding view is used, so that the information of the j-th view can be used. On the other hand, when the decrease in the tube voltage is large, the data of the corresponding view is corrected by the data of the previous and subsequent views, so that the artifact due to the abnormal discharge can be reduced.
[0009]
According to a third aspect, the present invention relates to an X-ray CT apparatus having the above-described configuration, which detects a decrease in the reference channel data but does not detect a decrease in the tube current. And a data correction method characterized by correcting the following.
In the data correction method according to the third aspect, since the X-ray is impeded from entering the reference channel, if the reference channel data is used as it is, inappropriate reference correction will be performed. Therefore, the reference channel data is corrected. Thereby, appropriate reference correction can be performed.
[0010]
According to a fourth aspect, in the data correction method having the above configuration, when correcting the reference channel data, the reference channel data of the corresponding view is discarded and the reference channel data corresponding to the actually measured tube current is used. A data correction method is provided.
In the data correction method according to the fourth aspect, since the X-rays are impeded from entering the reference channel, if the reference channel data is used as it is, inappropriate reference correction is performed. Therefore, the reference channel data of the corresponding view is discarded, and the reference channel data corresponding to the actually measured tube current is used. Thereby, appropriate reference correction can be performed.
[0011]
In a fifth aspect, the present invention provides a means for monitoring reference channel data and a tube voltage, a method for detecting a decrease in the reference channel data and a decrease in the tube voltage, and a method for detecting a decrease in the reference channel data and detecting a previous abnormal discharge. If the state is in the abnormal state, it is determined that the discharge is abnormal, and the decrease in the reference channel data is detected, but the decrease in the tube current is not detected. An X-ray CT apparatus characterized by comprising:
In the X-ray CT apparatus according to the fifth aspect, not only the first condition that “reference channel data decreases” but also the second condition that “tube voltage decreases” or “the last time was in an abnormal discharge state” Therefore, the abnormal discharge and the reference channel disturbance can be properly distinguished.
[0012]
According to a sixth aspect of the present invention, in the X-ray CT apparatus having the above-described configuration, a beam hardening correction unit configured to perform beam hardening correction using an actually measured tube voltage when an abnormal discharge is determined; An X-ray CT apparatus comprising: an abnormal discharge correction unit that corrects data of a corresponding view based on data of the previous and subsequent views when the data is large.
In the X-ray CT apparatus according to the sixth aspect, the data correction method according to the second aspect can be suitably implemented. Further, since the beam hardening correction is performed using the actually measured tube voltage, the beam hardening correction can be performed more appropriately than the beam hardening correction using the tube voltage set by the operator.
[0013]
According to a seventh aspect, the present invention provides the X-ray CT apparatus having the above-mentioned configuration, further comprising: reference channel data correction means for correcting reference channel data when it is determined that reference channel interference has occurred. I will provide a.
In the X-ray CT apparatus according to the seventh aspect, the data correction method according to the third aspect can be suitably implemented.
[0014]
According to an eighth aspect of the present invention, in the X-ray CT apparatus having the above-mentioned configuration, the reference channel data correction unit discards reference channel data of a corresponding view and outputs a reference channel corresponding to an actually measured tube current. An X-ray CT apparatus using data is provided.
The X-ray CT apparatus according to the eighth aspect can suitably implement the data correction method according to the fourth aspect.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to the embodiments shown in the drawings. Note that the present invention is not limited to this.
[0016]
FIG. 1 is a block diagram of an X-ray CT apparatus according to one embodiment of the present invention.
The X-ray CT apparatus 100 includes an operation console 1, an imaging table 10, and a scanning gantry 20.
[0017]
The operation console 1 includes an input device 2 that receives input from an operator, a central processing unit 3 that executes data correction processing and the like according to the present invention, a data collection buffer 5 that collects data acquired by the scanning gantry 20, and a projection device. The apparatus includes a CRT 6 that displays a CT image reconstructed from data, and a storage device 7 that stores programs, data, and X-ray CT images.
[0018]
The imaging table 10 includes a cradle 12 for placing a subject thereon and taking it into and out of a bore (cavity) of a scanning gantry 20. The cradle 12 is driven by a motor built in the photographing table 10.
[0019]
The scanning gantry 20 rotates the X-ray tube 21, the X-ray controller 22, the collimator 23, the detector 24, the DAS (Data Acquisition System) 25, and the X-ray tube 21 around the body axis of the subject. A rotation controller 26 to be operated and a control interface 29 for exchanging control signals and the like with the operation console 1 and the imaging table 10 are provided.
Note that the end channel among the many channels of the detector 24 is a reference channel.
[0020]
FIG. 2 is a flowchart showing the flow of the operation of the X-ray CT apparatus 100.
In step S1, while the X-ray tube 21 and the detector 24 are rotated around the imaging target by the scanning gantry 20, data Dat_i, j and reference channel data represented by a channel number i and a view number j for each view. Collect Ref_j. At this time, the X-ray tube 21 is driven by the tube voltage and tube current set by the operator in advance. However, when abnormal discharge occurs, the tube voltage and tube current fluctuate. Perform actual measurements.
[0021]
In step S2, preprocessing (offset correction, logarithmic conversion, reference correction, sensitivity correction, etc.) is performed. The main part of this preprocessing will be described later with reference to FIG.
[0022]
In step S3, an image reconstruction process is performed on the preprocessed data Dat_i, j to generate a CT image.
In step S4, post-processing is performed on the CT image to obtain a CT image suitable for display.
In step S5, a CT image is displayed on CRT6.
[0023]
FIG. 3 is a flowchart showing main steps of the pre-processing S2.
In step P1, the reference channel data of the j-th view is set to Ref_j, the threshold coefficient for reference channel data is set to Thref (0 <Thref <1, for example, Thref = 0.5), and the reference channel data in the previous normal state is set. When Old_Ref is used,
Ref_j ≦ Thref · Old_Ref
Is satisfied, that is, if the X-ray intensity drops abruptly, it is determined that abnormal discharge or reference channel disturbance has occurred, and the routine goes to Step P2. If not, it is determined that neither abnormal discharge nor reference channel interference has occurred, and the routine goes to Step P8.
[0024]
In step P2, the tube voltage of the j-th view is set to kV_j, the threshold voltage coefficient for the tube voltage is set to Th1 kV (0 <Th1 kV <1, for example, Th1 kV = 0.8), and the tube voltage in the previous normal state is set to Old_kV. When
kV_j ≦ Th1 kV · Old_kV
Is satisfied, that is, if the tube voltage drops abruptly, it is determined that abnormal discharge has occurred, and the routine goes to Step P3. If the previous view is in the abnormal discharge state, it is determined that recovery from the abnormal discharge has not yet occurred, and the process proceeds to Step P3. If none of these holds, it is determined that reference channel interference has occurred, and the routine goes to Step P5.
[0025]
In step P3, when the abnormal discharge degree determination threshold coefficient is set to Th2 kV (0 <Th2 kV <1, for example, Th2 kV = 0.7),
kV_j ≦ Th2 kV · Old_kV
Holds, it is determined that the degree of abnormal discharge is large, and the routine goes to Step P4. If this is not the case, it is determined that the degree of abnormal discharge is small or the abnormal discharge has been recovered to some extent, so that step P4 is skipped to use the data Dat_i, j as it is, and the process proceeds to step P8.
[0026]
In Step P4, the data Dat_i, j of the j-th view of the i-th channel is corrected by the data Dat_i, j-2, Dat_i, j-1, Dat_i, j + 1, Dat_i, j + 2 of the i-th channel of the preceding and following views. . That is,
Dat_i, j ← (α2 · Dat_i, j−2 + α1 · Dat_i, j−1 + β · Dat_i, j + γ1 · Dat_i, j + 1 + γ2 · Dat_i, j + 2) / (α2 + α1 + β + γ1 + γ2)
0 <α2, α1, β, γ1, γ2
This is the abnormal voltage correction (Spitz correction).
Here, since a long abnormal discharge lasts for about two views, by using data for two views before and after, it is possible to always include two or more data of normal views. On the other hand, when data for one view before and after is used as in the related art, only one piece of data for a normal view may be used.
Also, β = 0 may be set.
Thereafter, the process proceeds to Step P8.
[0027]
In Step P5, reference correction is performed using the reference channel data corresponding to the actually measured tube current mA. For example, assuming that the tube current mA_j in the j-th view is “200”, the reference channel data Ref of the normal view in which the actually measured tube current mA is “200” is searched, and the reference channel data Ref_j is used. To correct the reference. This is because the value can be estimated as a value when no reference channel interference occurs.
A value obtained by correcting the reference channel data Ref_j of the j-th view by reference channel data Ref_j−1 and Ref_j + 1 of the previous and subsequent views, that is,
Ref_j ← (a · Ref_j−1 + b · Ref_j + c · Ref_j + 1) / (a + b + c)
0 <a, b, c
May be used to perform reference correction.
Thereafter, the process proceeds to Step P9.
[0028]
In Step P8, reference correction is performed using the reference channel data Ref_j. This is because, in the case of abnormal discharge, the X-ray dose decreases in any channel, and the reduced X-ray dose is represented by the reference channel data Ref_j.
Thereafter, the process proceeds to Step P10.
[0029]
In step P9, BH correction is performed using the tube voltage set by the operator in advance.
Thereafter, the process proceeds to steps after step P10 (not shown).
[0030]
In Step P10, BH correction is performed using the actually measured tube voltage. That is, at the time of abnormal discharge, since the actual tube voltage is lower than the tube voltage set by the operator, the BH correction using the actually measured tube voltage can perform more appropriate correction.
Thereafter, the process proceeds to subsequent steps (not shown).
[0031]
According to the above X-ray CT apparatus 100, the following effects can be obtained.
(1) The data Dat_i, j can be corrected by distinguishing between abnormal discharge (spits) occurring in the X-ray tube 21 and interference (ref. Obstruction) of X-rays incident on the reference channel of the detector 24.
(2) When the degree of abnormal discharge is small, the data Dat_i, j is used as it is, so that the information of the j-th view can be utilized.
(3) When the degree of the abnormal discharge is large, the data Dat_i, j is corrected using the data of the previous and subsequent normal views, so that an artifact can be reduced as compared with the case where the data Dat_i, j is used as it is.
(4) When an abnormal discharge occurs, the BH correction is performed using the actually measured tube voltage without using the tube voltage set by the operator, so that more appropriate BH correction can be performed.
(5) When reference channel interference occurs, normal reference channel data corresponding to the actually measured tube current mA is used without using the reference channel data Ref_j as it is, or reference channel data of a normal view before and after is used. Then, the reference channel data Ref_j is corrected, so that more appropriate reference correction can be performed.
[0032]
【The invention's effect】
According to the data correction method and the X-ray CT apparatus of the present invention, abnormal data (spits) occurring in an X-ray tube and X-ray incident interference (ref. Obstruction) on a reference channel of a detector are distinguished from each other. Can be corrected.
[Brief description of the drawings]
FIG. 1 is a block diagram of an X-ray CT apparatus according to one embodiment of the present invention.
FIG. 2 is a flowchart showing a flow of an operation of the X-ray CT apparatus according to one embodiment of the present invention.
FIG. 3 is a flowchart showing main steps of preprocessing of the X-ray CT apparatus according to one embodiment of the present invention.
[Explanation of symbols]
21 X-ray tube 22 Detector 100 X-ray CT apparatus

Claims (8)

Monitor the reference channel data and tube voltage with an X-ray CT device, and detect a decrease in reference channel data, but do not detect a decrease in tube current and do not perform abnormal discharge correction if the previous discharge was not abnormal. A data correction method characterized in that: 2. The data correction method according to claim 1, wherein a decrease in the tube voltage is detected when a decrease in the reference channel data and a decrease in the tube voltage are detected, and when a decrease in the reference channel data is detected and the previous discharge was abnormal. A data correction method comprising: using the data of a corresponding view when is small, and correcting the data of the corresponding view with the data of the previous and subsequent views when the drop in tube voltage is large. 2. The X-ray CT apparatus according to claim 1, wherein a decrease in reference channel data is detected, but a decrease in tube current is not detected, and the reference channel data is corrected when no abnormal discharge state was detected last time. A data correction method characterized in that: 3. The data correction method according to claim 1, wherein when the reference channel data is corrected, the reference channel data of the corresponding view is discarded and the reference channel data corresponding to the actually measured tube current is used. Method. Means for monitoring the reference channel data and the tube voltage; determining abnormal discharge when detecting a decrease in the reference channel data and a decrease in the tube voltage; and detecting a decrease in the reference channel data and in a case where the previous discharge was in an abnormal discharge state. And X-ray CT which comprises means for detecting a decrease in the reference channel data but not detecting a decrease in the tube current, and determining that the reference channel has been disturbed when the abnormal discharge state has not been detected last time. apparatus. In the X-ray CT apparatus having the above-described structure, a beam hardening correction unit that corrects beam hardening using an actually measured tube voltage when an abnormal discharge is determined, and a corresponding view when a decrease in the tube voltage is large. An X-ray CT apparatus comprising: an abnormal discharge correction unit that corrects data based on data of views before and after the abnormal discharge. 7. The X-ray CT apparatus according to claim 5, further comprising reference channel data correction means for correcting reference channel data when it is determined that reference channel interference has occurred. 7. The X-ray CT apparatus according to claim 1, wherein said reference channel data correcting means discards reference channel data of a corresponding view and uses reference channel data corresponding to an actually measured tube current. Line CT device.

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