JP4535600B2 - Collimator tracking control method and X-ray CT apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a collimator tracking control method and an X-ray CT (Computed Tomography) apparatus. More specifically, the present invention relates to X-rays incident on a reference channel so that the relative positions of a collimator and a multi-X-ray detector do not fluctuate. The present invention relates to a collimator tracking control method and an X-ray CT apparatus that can perform tracking control of a collimator so as not to be inappropriate even when blocked.
[0002]
[Prior art]
FIG. 6 is a schematic diagram showing an X-ray tube, a collimator, and a twin X-ray detector.
The first detector row 27A includes N channels from the first channel 27A1 to the Nth channel 27AN, a left reference channel 27AL arranged outside the first channel 27A, and an outside of the Nth channel 27AN. The right reference channel 27AR is provided, and these channels are arranged in an arc shape.
Similarly, the second detector row 27B is also provided with N channels from the first channel to the Nth channel, the left reference channel 27BL arranged outside the first channel, and outside the Nth channel. Right reference channel 27BR, and these channels are arranged in an arc.
[0003]
The outputs of the reference channels 27AL, 27AR, 27BL and 27BR are used for various corrections. The reason why these reference channels 27AL, 27AR, 27BL, and 27BR are arranged at the ends of the detector rows 27A and 27B is that X-rays incident on them are not blocked by the subject.
[0004]
The X-ray Io emitted from the X-ray tube 21 passes through the aperture S of the collimator 23 to become a flat X-ray beam Xr, and the first detector row 27A and the second detector row 27B of the twin X-ray detector 27. Is incident on.
The boundary virtual line Po is a virtual line indicating the boundary of the X-ray beam Xr that is incident on the first detector row 27A and the portion that is incident on the second detector row 27B.
The width of the X-ray beam Xr in the isocenter IC that is the rotation center when at least one of the X-ray tube 21 and the twin X-ray detector 27 rotates is referred to as an X-ray beam width Xo. Further, the width of the X-ray beam width Xo that is incident on the first detector row 27A is referred to as a first slice thickness d1. Further, the width corresponding to the incident on the second detector row 27B is referred to as a second slice thickness d2.
[0005]
The first slice thickness d1 and the second slice thickness d2 are defined by the relative positions of the collimator 23 and the twin X-ray detector 27 and the aperture width of the aperture S.
[0006]
[Problems to be solved by the invention]
The relative positions of the collimator 23 and the twin X-ray detector 27 need to be maintained so as not to fluctuate during scanning. For this purpose, the position of the collimator 23 should be controlled so that the ratio of the total output value of the reference channels 27AL and 27AR and the total output value of the 27BL and 27BR becomes the target value without fluctuation during the scan. .
[0007]
However, if the position of the subject relative to the isocenter IC is not appropriate, the subject K blocks X-rays incident on the reference channels 27AL, 27AR, 27BL, and 27BR at a certain position as shown in FIG. Even if it is not, X-rays incident on the reference channels 27AL, 27AR, 27BL, and 27BR at the half-rotated position shown in FIG. In this case, the follow-up control based on the outputs of the reference channels 27AL, 27AR, 27BL, and 27BR becomes inappropriate, and there is a problem that the relative position error may increase.
[0008]
Therefore, an object of the present invention is to follow-up control the collimator so that the relative position between the collimator and the multi-X-ray detector does not fluctuate and the X-ray incident on the reference channel is not improper even when the subject is obstructed. Another object of the present invention is to provide a collimator tracking control method and an X-ray CT apparatus that can perform the same.
[0009]
[Means for Solving the Problems]
In a first aspect, the present invention provides (1) one or more channels at one end or both ends of a detector row at the first end of a multi-X-ray detector having a plurality of rows of detector rows in which a large number of channels are arranged. As a first reference channel, one or a plurality of channels at one or both ends of the detector row at the second end of the multi-X-ray detector is used as a second reference channel, and an X-ray tube and a multi-X-ray detector around the isocenter When at least one of the first and second reference channels rotates, the position of the collimator is controlled to follow the target value without changing the output ratio of the first reference channel and the second reference channel, and (2) the first reference channel and the second reference The follow-up control is stopped when the sum of the channel outputs tends to decrease and the difference between the output of the first reference channel and the second reference channel exceeds the stop threshold, and (3) the follow-up control is stopped. The first reference channel in the state Le and the difference between the output of the second reference channels resumes the tracking control when it becomes below resume threshold, providing a collimator tracking control method characterized by.
In the collimator tracking control method according to the first aspect, tracking control of the collimator is performed by (1) so that the relative position of the collimator and the multi-X-ray detector does not fluctuate.
Further, the above (2) prevents the relative position error from becoming larger due to the follow-up control. That is, the tracking control is continued when the subject enters the X-ray path incident on the reference channel and the difference between the outputs of the first reference channel and the second reference channel exceeds the stop threshold. If so, the error in the relative position may increase, and the follow-up control is stopped. If the sum of the outputs of the first reference channel and the second reference channel does not tend to decrease, the subject does not enter the X-ray path incident on the reference channel. The tracking error is not stopped because the position error does not increase. Further, if the difference between the outputs of the first reference channel and the second reference channel is smaller than the stop threshold, the error in the relative position does not increase even if the follow-up control is continued, so the follow-up control is not stopped.
Further, the follow-up control of the collimator is restarted by the above (3). That is, if the difference between the outputs of the first reference channel and the second reference channel is smaller than the restart threshold, the error in the relative position does not increase even when the tracking control is restarted, and the tracking control is restarted.
The restart threshold is made smaller than the stop threshold to avoid hunting.
[0010]
According to a second aspect, the present invention provides a collimator tracking control method configured as described above, wherein instead of (3), (3) the outputs of the first reference channel and the second reference channel in a state where tracking control is stopped. A collimator follow-up control method is provided, wherein the follow-up control is restarted when the difference between the values becomes equal to or less than a restart threshold value or when the follow-up control is half-turned from stopping.
In the collimator tracking control method according to the second aspect, if the difference between the outputs of the first reference channel and the second reference channel is smaller than the restart threshold, the error in the relative position does not increase even when tracking control is restarted. Resume control. In addition, even if X-rays incident on the reference channel at a certain position are blocked by the subject, they often return to a position where they are not blocked at the half-turned position. Resume control.
[0011]
According to a third aspect, in the collimator tracking control method having the above configuration, the present invention provides the tracking control from the start of tracking control until the difference between the outputs of the first reference channel and the second reference channel is equal to or less than a determination start threshold. A collimator follow-up control method characterized by not stopping is provided.
When the tracking control is started, the sum of the outputs of the first reference channel and the second reference channel tends to decrease even if the X-ray incident on the reference channel is not blocked by the subject, and the first reference channel The output difference of the second reference channel may be equal to or greater than the stop threshold value. In this case, the follow-up control should not be stopped.
Therefore, in the collimator tracking control method according to the third aspect, the tracking control is not stopped until the difference between the output of the first reference channel and the second reference channel becomes equal to or less than the threshold value for determination start from the start of the tracking control.
The discrimination start threshold value is set to be smaller than the stop threshold value so that the first follow-up control is not interrupted until the follow-up control effect is obtained.
[0012]
In a fourth aspect, the present invention is an X-ray CT apparatus having a multi-X-ray detector having a plurality of detector rows in which a large number of channels are arranged, and (1) a first end of the multi-X-ray detector. One or a plurality of channels at one or both ends of the detector row of the multi-ray detector are used as a first reference channel, and one or a plurality of channels at one or both ends of the detector row at the second end of the multi-X-ray detector are a second reference channel. And the position of the collimator so that the ratio of the output of the first reference channel and the second reference channel does not change and becomes a target value when at least one of the X-ray tube and the multi-X-ray detector rotates around the isocenter. (2) The difference between the outputs of the first reference channel and the second reference channel is equal to or greater than the stop threshold value, and the sum of the outputs of the first reference channel and the second reference channel tends to decrease. Sometimes following above Follow-up control stop means for stopping control, and (3) the follow-up control is resumed when the difference between the outputs of the first reference channel and the second reference channel is equal to or less than the restart threshold value in the state where the follow-up control is stopped. An X-ray CT apparatus comprising: a tracking control restarting unit.
In the X-ray CT apparatus according to the fourth aspect, the collimator tracking control method according to the first aspect can be suitably implemented.
[0013]
In a fifth aspect, the present invention provides an X-ray CT apparatus having the above-described configuration, in which the difference between outputs of the first reference channel and the second reference channel is stopped in a state where tracking control is stopped instead of the tracking control restarting means. There is provided an X-ray CT apparatus comprising a follow-up control resuming means for resuming the follow-up control when is less than a restart threshold or when the follow-up control is half-turned from stopping.
In the X-ray CT apparatus according to the fifth aspect, the collimator tracking control method according to the second aspect can be suitably implemented.
[0014]
In a sixth aspect, the present invention provides the X-ray CT apparatus having the above-described configuration, wherein the tracking control stop means determines that a difference between outputs of the first reference channel and the second reference channel from the start of tracking control is a threshold value for determining start. Provided is an X-ray CT apparatus characterized in that tracking control is not stopped until the following occurs.
In the X-ray CT apparatus according to the sixth aspect, the collimator tracking control method according to the third aspect can be suitably implemented.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to embodiments shown in the drawings. Note that the present invention is not limited thereby.
[0016]
FIG. 1 is a block diagram of an X-ray CT apparatus 100 according to an 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 console 1 includes an input device 2 that receives an operator's instruction input and information input, and a central processing device 3 that executes a scan process, various correction processes, an image reconstruction process, a collimator tracking control process according to the present invention, and the like. The control interface 4 for exchanging control signals with the imaging table 10 and the scanning gantry 20, the data collection buffer 5 for collecting data acquired by the scanning gantry 20, and the X-ray image reconstructed from the data are displayed. A CRT 6 and a storage device 7 for storing programs, data, and X-ray images are provided.
[0018]
The scanning gantry 20 includes an X-ray tube 21, an X-ray controller 22, a collimator 23, a collimator controller 24, a rotation controller 26 for rotating the X-ray tube 21 and the like around an isocenter (IC in FIG. 2), And a twin X-ray detector 27 having two detector rows.
[0019]
FIG. 2 is a schematic diagram showing the X-ray tube 21, the collimator 23, and the twin X-ray detector 27.
The X-ray Io emitted from the X-ray tube 21 passes through the aperture S of the collimator 23 to become a flat X-ray beam Xr, and the first detector row 27A and the second detector row 27B of the twin X-ray detector 27. Is incident on.
The collimator controller 24 adjusts the opening width and position of the aperture S of the collimator 23 based on a command from the central processing unit 3.
The boundary virtual line Po is a virtual line indicating the boundary of the X-ray beam Xr that is incident on the first detector row 27A and the portion that is incident on the second detector row 27B.
The width of the X-ray beam Xr in the isocenter IC is referred to as an X-ray beam width Xo. Further, of the X-ray beam width Xo, the width incident on the first detector row 27A is the first slice thickness d1, and the width incident on the second detector row 27B is the second slice thickness d2. is there.
[0020]
The first detector row 27A includes N channels from the first channel 27A1 to the Nth channel 27AN, a first detector row left reference channel 27AL disposed outside the first channel 27A, and an Nth channel. The first detector row right reference channel 27AR is arranged outside the 27AN, and these channels are arranged in an arc.
Similarly, the second detector row 27B includes N channels from the first channel to the Nth channel, a second detector row left reference channel 27BL disposed outside the first channel, and an Nth channel. And the second detector row right reference channel 27BR arranged outside, and these channels are arranged in an arc shape.
[0021]
FIG. 3 is a flowchart of the collimator tracking control process according to the embodiment of the present invention. This collimator tracking control process is activated simultaneously with the start of scanning.
[0022]
In step S1, when the X-ray tube 21 and the twin X-ray detector 27 rotate around the isocenter IC, the ratio of the outputs of the first detector row reference channels 27AL and 27AR and the second detector row reference channels 27BL and 27BR is The position of the collimator 23 is controlled to follow the target value without fluctuation.
Specifically, for example, the sum of the outputs of the first detector array reference channels 27AL and 27AR is A, the sum of the outputs of the second detector array reference channels 27BL and 27BR is B, and the target value of the ratio A / B Is α
Δ = (A−αB) / (A + αB)
Is controlled to follow the position of the collimator 23 so that becomes 0%.
The target value α is determined by the ratio of the first slice thickness d1 and the second slice thickness d2,
α = d1 / d2
It is.
[0023]
In step S2, the collimator tracking control process is terminated if the scan is completed, and the process proceeds to step S3 if the scan is not completed.
In step S3, the collimator follow-up control process is continued until the difference between the outputs of the first detector array reference channels 27AL and 27AR and the second detector array reference channels 27BL and 27BR becomes equal to or less than the determination start threshold value. If it is equal to or less than the threshold, the process proceeds to step S4.
Specifically, for example, when the threshold value for starting discrimination is “5”%,
| A−αB | / (A + αB) ≦ 0.05
If so, the process proceeds to step S4. If not, the process returns to step S2.
[0024]
In step S4, if the sum (A + B) of the outputs of the first detector array reference channels 27AL and 27AR and the second detector array reference channels 27BL and 27BR is not decreasing, the process proceeds to step S5, and if it is decreasing, step S6 is performed. Proceed to
Specifically, for example, the sum of the sum (A + B) from the current view to the past 15 views is divided by the sum of the sum (A + B) from the previous view to the past 15 views, and the quotient is “ If it is not "1 or less", the process proceeds to step S5, and if "1 or less", the process proceeds to step S6.
[0025]
In step S5, the collimator tracking control process is terminated if the scan is completed, and the process returns to step S4 if the scan is not completed.
[0026]
In step S6, if the difference between the outputs of the first detector array reference channels 27AL and 27AR and the second detector array reference channels 27BL and 27BR is not equal to or greater than the stop threshold value, the process returns to step S5. move on.
Specifically, for example, when the stop threshold is “10”%,
| A−αB | / (A + αB) ≧ 0.10
If not, the process returns to step S5, and if so, the process proceeds to step S7.
In step S7, the follow-up control is stopped.
[0027]
In step S8, if the difference between the outputs of the first detector array reference channels 27AL, 27AR and the second detector array reference channels 27BL, 27BR is less than or equal to the restart threshold value, the process proceeds to step S9. If not, the process proceeds to step S10. .
Specifically, for example, when the restart threshold is set to “5”%,
| A−αB | / (A + αB) ≦ 0.05
If so, the process proceeds to step S9, and if not, the process proceeds to step S10.
[0028]
In step S9, the follow-up control is resumed. Then, the process returns to step S4.
[0029]
In step S10, it is checked whether the follow-up control has been half-rotated from the position where it has been stopped. If half-rotation has occurred, the process returns to step S9. If not, the process proceeds to step S11.
[0030]
In step S11, the collimator tracking control process is terminated if the scan is completed, and the process returns to step S8 if the scan is not completed.
[0031]
FIG. 4 is a time chart showing an example of the progress of the collimator tracking control process.
The tracking control is continued until | A−αB | / (A + αB) becomes equal to or less than the discrimination start threshold from the start of the scan, that is, the tracking control.
When | A−αB | / (A + αB) is equal to or less than the threshold value for determination start, the determination operation, that is, the operations in steps S4 to S6, is started.
When Σ (A + αB) tends to decrease and | A−αB | / (A + αB) becomes equal to or greater than the stop threshold value, the follow-up control is stopped.
When | A−αB | / (A + αB) is equal to or less than the restart threshold, the tracking control is restarted.
[0032]
FIG. 5 is a time chart showing another example of the collimator tracking control process.
The tracking control is continued until | A−αB | / (A + αB) becomes equal to or less than the discrimination start threshold from the start of the scan, that is, the tracking control.
When | A−αB | / (A + αB) is equal to or less than the threshold value for determination start, the determination operation, that is, the operations in steps S4 to S6, is started.
When Σ (A + αB) tends to decrease and | A−αB | / (A + αB) becomes equal to or greater than the stop threshold value, the follow-up control is stopped.
If the halfway rotation is made from the position where the tracking control is stopped, the tracking control is resumed.
[0033]
According to the X-ray CT apparatus 100 described above, the relative position between the collimator 23 and the twin X-ray detector 27, that is, the relative position between the X-ray beam Xr and the twin X-ray detector 27 is not changed, and the reference channel 27AL, The collimator 23 can be subjected to follow-up control so that X-rays incident on the 27AR, 27BL, and 27BR do not become inappropriate even when the subject K is blocked by the subject K.
[0034]
Even when an X-ray detector having three or more detector rows is used, if the collimator follow-up control process is performed in the same manner as described above for two detector rows corresponding to both ends of the X-ray beam Xr in the width direction, The invention can be applied.
[0035]
【The invention's effect】
According to the collimator tracking control method and the X-ray CT apparatus of the present invention, the relative position between the collimator and the multi-X-ray detector does not fluctuate, and even when X-rays incident on the reference channel are blocked by the subject. The collimator can be tracked and controlled so as not to occur.
[Brief description of the drawings]
FIG. 1 is a block diagram of an X-ray CT apparatus according to an embodiment of the present invention.
FIG. 2 is a schematic diagram showing an X-ray tube, a collimator, and a twin X-ray detector.
FIG. 3 is a flowchart showing collimator tracking control processing according to an embodiment of the present invention.
FIG. 4 is a time chart showing an example of the progress of a collimator tracking control process.
FIG. 5 is a time chart showing another example of the collimator tracking control process.
FIG. 6 is a schematic diagram showing an X-ray tube, a collimator, and a twin X-ray detector.
FIG. 7 is an explanatory diagram showing a case where X-rays incident on a reference channel are blocked by a subject depending on the rotational positions of an X-ray tube and an X-ray detector.
[Explanation of symbols]
3 Central Processing Unit 4 Control Interface 20 Scanning Gantry 21 X-ray Tube 23 Collimator 24 Collimator Controller 27 Twin X-ray Detector 27A First Detector Row 27AL First Detector Row Left Reference Channel 27AR First Detector Row Right Reference Channel 27B Second detector row 27BL Second detector row left reference channel 27BR Second detector row right reference channel 100 X-ray CT apparatus S Aperture

Claims (6)

(1) One or a plurality of channels at one end or both ends of a first detector row of a multi-X-ray detector having a plurality of detector rows in which a large number of channels are arranged are used as a first reference channel, and the multi-X-rays One or more channels at one or both ends of the detector row at the second end of the detector are used as the second reference channel, and the first reference is made when at least one of the X-ray tube and the multi-X-ray detector rotates around the isocenter. Follow-up control of the collimator position so that the ratio of the output of the channel and the second reference channel does not change and becomes the target value,
(2) The follow-up control is stopped when the sum of the outputs of the first reference channel and the second reference channel tends to decrease and the difference between the outputs of the first reference channel and the second reference channel is equal to or greater than the stop threshold. ,
(3) When the follow-up control is stopped, the follow-up control is resumed when the difference between the outputs of the first reference channel and the second reference channel is equal to or less than the restart threshold.
A collimator tracking control method characterized by the above. In the collimator tracking control method according to claim 1, instead of (3),
(3) The tracking control is restarted when the difference between the outputs of the first reference channel and the second reference channel is equal to or less than the restart threshold value or when the tracking control is stopped by a half rotation.
A collimator tracking control method characterized by the above. 3. The collimator tracking control method according to claim 1, wherein the tracking control is not stopped until the difference between the output of the first reference channel and the second reference channel is equal to or less than a threshold value for determination start after the tracking control is started. A collimator tracking control method characterized by the above. An X-ray CT apparatus having a multi-X-ray detector having a plurality of detector rows in which a large number of channels are arranged,
(1) One or a plurality of channels at one end or both ends of the detector row at the first end of the multi X-ray detector is used as a first reference channel, and one end of the detector row at the second end of the multi X-ray detector or One or more channels at both ends are used as the second reference channel, and when at least one of the X-ray tube and the multi-X-ray detector rotates around the isocenter, the output ratio of the first reference channel and the second reference channel varies. Follow-up control means for following-controlling the position of the collimator so that the target value is reached,
(2) The follow-up control is stopped when the sum of the outputs of the first reference channel and the second reference channel tends to decrease and the difference between the outputs of the first reference channel and the second reference channel is equal to or greater than the stop threshold. Following control stop means;
(3) Follow-up control restarting means for restarting the follow-up control when the difference between the outputs of the first reference channel and the second reference channel is equal to or less than the restart threshold value in a state in which the follow-up control is stopped;
An X-ray CT apparatus comprising: 5. The X-ray CT apparatus according to claim 4, wherein a difference between outputs of the first reference channel and the second reference channel is equal to or less than a restart threshold value when tracking control is stopped instead of the tracking control restarting unit. Or an X-ray CT apparatus comprising a follow-up control restarting unit that restarts the follow-up control when the follow-up control is stopped by a half rotation. 6. The X-ray CT apparatus according to claim 4, wherein the follow-up control stop means has a difference in output between the first reference channel and the second reference channel that is equal to or less than a determination start threshold value from the start of follow-up control. The X-ray CT apparatus is characterized in that the tracking control is not stopped until.

Images