JP4245178B2 - X-ray CT system - Google Patents

Description

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

There is Patent Document 1 relating to X-ray beam shift correction. This conventional example detects unnecessary movement of the fan beam position in the body axis direction (which may be regarded as movement of the focal point) due to thermal drift and mechanical displacement of the X-ray tube, and cancels out the unnecessary movement. The collimator was moved so that the correct scanning could be performed.
JP-A-4-227238

  In addition to the conventional example, there is a patent application (Japanese Patent Application No. 7-302502) filed by the present applicant regarding X-ray beam shift correction. This prior application has a mechanism for moving the X-ray tube. FIG. 2 includes an X-ray tube 1, collimators 3a and 3b, a multi-channel detector 4a, a focus movement detector 4b, an X-ray tube drive mechanism 8, a comparator 5, a drive control device 7, and amplifiers 6a and 6b. The X-ray tube 1 has a rotating anode 1a and a filament 1b, and X-rays are emitted from the rotating anode 1a. Of the X-rays, the main X-ray beam 2a is an X-ray used for original imaging, and the sub-X-ray beam 2b is used for shift correction. The collimator 3a controls the main X-ray aperture, and the collimator 3b controls the sub-X-ray aperture.

The focal point movement detector 4b has the detection configuration shown in FIG. When the sub-X-ray beam 2b is incident on the detector 4b having the oblique mask covered with the oblique mask, it is converted into an electrical signal and signals A and B are output. At this time, if the X-ray beam is on the moving direction A side, A> B, and if the X-ray beam is on the moving direction B side, A <B. The signals A and B are input to the comparator 5 via the amplifiers 6a and 6b (or the outputs of the amplifiers 6a and 6b may be A and B). The comparator 5 compares the magnitudes of the signals A and B, and if necessary, the movement signal 9 is output to the drive control device 7 to drive the tube 1 so as to correct the position of the fan beam. While X-rays are being emitted, this system is controlled so that the output signal of the comparator is A = B.
By performing the X-ray beam position shift correction in this way, a slice image free from ring artifacts is provided.

The prior art and the prior application have a problem that ring artifacts occur in the following cases.
(1) The waiting time until the next patient is examined after the lunch break is long, and the system of the X-ray CT system cools down.
(2) When the scanner is tilted.
(3) When the scanner is returned from the tilted state.
This is because the distance from the fan beam position before the shift correction to the regular position after the shift correction is large, and it takes time to perform the correction, and a ring artifact occurs during this time.

  FIG. 4 shows an explanatory diagram thereof. For example, in an example in which a 1-second scan is performed after the scan is paused, the focus is shifted to a normal position by the focus shift correction operation during the 1-second scan. However, if the moving distance is large, it takes time to move the focal point, and the shift is performed with a certain time delay as shown in FIG. In the process of the time delay, ring artifacts are generated because the regular position has not been reached.

  Furthermore, the above (1) to (3) will be specifically described. The focus shift during normal routine inspection (during repeated X-ray CT measurements) has a smaller amount of movement than this, so correction works effectively. However, in the case of (1), the temperature of the X-ray tube is cooled and lowered from the high state. Therefore, the anode in the X-ray tube is reduced from the thermally expanded state, and the focal point moves. The thermal drift of the focal point in the X-ray tube is used within 0.3mm in the axial direction. For this reason, the maximum movement amount during shift correction is 0.3 mm. In the case of (2), the focus shift due to mechanical stress applied to the X-ray tube itself and its pedestal during tilting (specifically, mechanical stress applied to the pedestal itself caused by gravity and centrifugal force) is the cause. . The graph of FIG. 5 shows the focus shift when returning from the tilted state to the upright state. It has been confirmed that a focal point shift of about 0.2 mm occurs when tilted upright from erect. In the case of (3), since there is a movable part of the ball bearing part in the anode support part inside the X-ray tube, there is play (play). That is, since the anode is a rotary anode, it is cantilevered by a bearing. Since it is not fixed, play occurs. As shown in the graph of FIG. 5, even if the scanner is returned to the erect state from the forward tilt state, the focus position is offset by about 0.1 mm and does not return to the normal position. Therefore, when tilting from the forward tilt to the rear tilt side, a displacement of 0.1 mm or more is generated by (displacement due to tilt) + (displacement due to play).

The following methods can be considered as the above-mentioned countermeasures, but are not realistic.
(1) Use an X-ray tube with little thermal drift and mechanical stress displacement.
(2) Use of a drive mechanism that can move and stop the X-ray tube or collimator instantaneously.
(3) Use of a detector capable of detecting the focal position when X-ray irradiation is not performed.
The reason (1) is an increase in cost due to a decrease in the yield of usable X-ray tubes.
In the case of (2), the moving speed can be improved, but it is difficult to stop it instantaneously because of the transient response of the current drive mechanism itself. In short, the higher the moving speed, the more the kinetic energy increases and the longer it takes to stop it, the followability to the moving command decreases and the position accuracy deteriorates.
In the case of (3), it is difficult to incorporate a measuring device for detecting the displacement of the anode and the vacuum vessel inside the X-ray tube.

  The present invention has been made to solve these problems, and an object of the present invention is to provide an X-ray CT apparatus capable of obtaining a slice image in which ring artifacts are reduced or eliminated.

  In order to achieve the above object, an X-ray CT apparatus according to the present invention includes an X-ray tube for irradiating X-rays, and an X-ray tube and an X-ray tube disposed opposite to each other with the X-ray tube and a subject interposed therebetween. X-ray detector for detecting a line, scanner for rotatably supporting the X-ray tube and the X-ray detector, and X-ray irradiation moved by thermal expansion of the anode of the X-ray tube or tilt of the scanner A focus movement detector for detecting a position, a focus movement correction device for correcting the position of the X-ray tube in accordance with the detected X-ray irradiation position, and preliminary exposure for irradiating only the focus movement detector with X-rays In the X-ray CT apparatus provided with a control means for controlling irradiation, between the X-ray tube and the subject, between the position for shielding the X-rays irradiated from the X-ray tube and the retracted position The X-ray shielding shutter, which is movably provided by the camera, and the X-ray shielding shutter before the main scan are arranged. With the X-ray tube preliminarily exposed to the X-ray tube while being moved to the shielding position, the X-ray irradiation position at the time of preliminary exposure is detected by the focus movement detector, and based on this detection signal And a control device for controlling the operation of the focus movement correction device.

Also, an X-ray tube for irradiating X-rays, an X-ray detector arranged to face the X-ray tube and detecting X-rays irradiated from the X-ray tube, the X-ray tube and the X-ray detector A scanner that is rotatably supported, a focus movement detector that detects an X-ray irradiation position moved by thermal expansion of the anode of the X-ray tube or a tilt of the scanner, and the detected X-ray irradiation position according to the detected X-ray irradiation position In an X-ray CT apparatus comprising: a focus movement correction apparatus that corrects the position of the X-ray tube; and a control means that controls preliminary exposure that irradiates only the focus movement detector with X-rays. The implementation condition of the preliminary exposure is any one of (1) a resting state of the X-ray tube for a certain time or more, and (2) a state where the tilt angle of the scanner is different from the previous scan, pre of the performed pre-exposure, thus the X-ray tube to the focus movement correcting device by And controlling so as to perform moving just before the main scan.

  According to the present invention, the shift of the X-ray beam position is corrected, and it is possible to reduce or eliminate ring artifacts appearing in the slice image. In particular, by performing preliminary exposure, it becomes possible to use an X-ray tube with a large focal shift in the CT apparatus, and the cost can be reduced by improving the yield of the X-ray tube.

  Further, simply adding a shutter function to the collimator as in the third embodiment makes it easy to apply to a conventional product, and the cost can be reduced because of the addition only.

  According to the present invention, preliminary movement control is performed by performing preliminary exposure in which only the movement detector is irradiated with X-rays immediately before the main scan. Thus, focus shift correction is performed immediately before the main scan, thereby reducing the movement distance and movement time during the main scan.

Some ideas for realizing the preliminary movement are as follows.
(1) In the command from the console to the scanner, add processing for preliminary movement control to the scan sequence program. In this case, the following pre-exposure execution conditions are given to the process.
Pre-exposure should be performed when the X-ray tube has been dormant for a certain period of time, and then the main scan should be performed. This fixed time often varies depending on the type of X-ray tube.

When a scanner tilt angle different from the previous scan is set, pre-exposure is performed and then the main scan is performed.
(2) Ensure that the subject is not exposed during pre-exposure. One example is the addition of a shutter mechanism that blocks the main X-ray beam for pre-exposure. Control the shutter so that the shutter is activated by the preliminary exposure start command from the console and returns at the end.
(3) Control X-ray pre-exposure with a pre-exposure command from the console.
(4) In addition to the X-ray tube, the position and opening of the collimator may be controlled.

The details will be described below.
FIG. 6 shows an X-ray CT apparatus. This X-ray CT apparatus has an X-ray tube 1, collimators 3a and 3b, a focus movement detector 4b, a comparator 5, amplifiers 6a and 6b, a drive control device 7, and a drive mechanism 8. The X-ray tube 1 has a filament 1b and a rotating anode 1a, and X-rays are emitted from the rotating anode 1a. Of the emitted X-rays, the main X-ray beam 2a is used for CT measurement, and the secondary X-ray beam 2a is used for shift correction. The collimators 3a and 3b perform apertures for each incident X-ray.

  The focus movement detector 4b, the comparator 5, and the drive control device 7 drive the drive mechanism 8 so that the signal A = B, perform position control of the X-ray tube 1, and perform shift correction. The X-ray CT apparatus further includes a shutter 13, a shutter drive mechanism 12, a shutter control device 11, an operation console 10, an X-ray control device 14, and a processor (or computer) 10A. The device 11, the mechanism 12, and the shutter 13 serve as means for preventing exposure to the subject during preliminary exposure. The X-ray control device 14 is a means for instructing X-ray exposure at the time of preliminary exposure. The processor 10A has various CT measurement sequence programs, and in response to an instruction from the console 10, the processor 10A outputs a preliminary exposure command 10a, a shutter close command to the device 11, and an X-ray exposure to the device 14. Generate a command.

A flowchart of the preliminary exposure condition scan program is shown in FIG. First, scan conditions are set (flow F1), and then it is checked whether or not a preliminary exposure execution determination input is specified (flow F2). This input setting is made on the console 10. If the preliminary exposure input is set, the processor 10A generates a preliminary exposure command 10a (flow F3). In response to this, the shutter 13 is closed (shielded) (flow F4), and the preliminary exposure (flow F5) is controlled to the X-ray tube 1 via the X-ray controller 14. When the preliminary exposure ends, control is performed to return the shutter 13 to the original retracted position (flow F6).
Thereafter, the main scan is controlled (F7). The shift to the main scan is also performed by the determination check (F2).

During the preliminary exposure, the secondary X-ray beam 2b is incident only on the focus movement detector 4b, and the signals A and B are output. The comparator 5 compares the magnitude relationship of the signals A and B, and the movement signal 9 is output. The Upon receiving this movement signal, the drive control device 7 operates the drive mechanism 8 to move the X-ray tube 1. As a result, the X-ray beam position moves in the normal direction, and the secondary X-ray beam 2b enters the focal point movement detector 4b so that the signal A = B. When the signal becomes A = B, the movement signal 6 from the comparator 5 is not output and the movement of the X-ray tube 1 is stopped. When the preliminary exposure is completed, the shutter control device 11 operates the shutter driving mechanism 12 so as to open the shutter 13 (or return it to the original position). The material, shape, and size of the shutter are the same as those of the collimator because X-ray shielding is intended. For example, the shutter uses brass (copper and zinc alloy).
After the series of preliminary exposure operations is completed, the main scan is started. The arrangement of the shutter 13 is not limited to the lower side of the collimator 3a, but may be on the upper side.

   A second embodiment is shown in FIG. The difference from FIG. 6 is that the collimator 3a, not the X-ray tube 1, is used as the object of movement when correcting the focus shift. For this purpose, the collimator 3a is provided with a drive mechanism 8A and a drive control device 7A for driving the drive mechanism 8A. Further, the focal point movement detector 4b is disposed below the collimator 3a. However, it is desirable that the X-rays at the end of the fan beam are incident on the focus movement detector 4b and that the X-ray measurement field of view is not obstructed. The shutter 13 is disposed below the focal point movement detector 4b so that X-rays are blocked during preliminary exposure. Except for these differences, the operation is the same as that of the first embodiment.

  A third embodiment is shown in FIG. Instead of the collimator 3a, a collimator 3c having a function as the shutter 13 is installed. Further, a shutter drive mechanism 15 and a collimator control device 16 for driving the same are provided in place of the shutter drive mechanism 12 and the shutter control device 11. When the pre-exposure is set, the apparatus 16 and mechanism 15 move the collimator 3c to a position as shown in FIG. 8 to prohibit X-ray irradiation to the subject. In the main scan, the collimator 3c is moved in the direction of the arrow, and control to a position such as the collimator 3a in FIGS. As a result, the main scan can be realized in the state shown in FIGS.

  It is also possible to perform preliminary focus shift without inserting the subject during preliminary exposure. For example, it is performed immediately before the main scan. At this time, the shutter mechanism is not necessarily required. The detector 4b is only an example.

The processing flowchart of this invention is shown. The structural example figure of the shift correction system of the X-ray beam position by the X-ray tube movement of the conventional X-ray CT apparatus. Explanatory drawing of the signals A and B output from a focus movement detector. The graph showing the focus position under focus shift correction operation. The graph which shows the relationship between a tilt angle and a focus position. 1 is a configuration example diagram of a shift correction system for an X-ray beam position by X-ray tube movement of the X-ray CT apparatus of the present invention. The structural example figure of the shift correction system of the X-ray beam position by the collimator movement of the X-ray CT apparatus of this invention. 1 is a configuration example diagram of a shift correction system for an X-ray beam position by X-ray tube movement of the X-ray CT apparatus of the present invention.

Explanation of symbols

  1 X-ray tube, 2 X-ray beam, 3 collimator, 4 detector, 5 comparator, 6 movement signal, 7 drive control device, 8 drive mechanism, 9 console, 10 preliminary exposure command, 11 shutter control device, 12 Shutter drive device, 13 shutter, 14 X-ray control device, 15 collimator drive mechanism, 16 collimator control device

Claims (5)

An X-ray tube that emits X-rays;
An X-ray detector that detects X-rays that are disposed opposite to each other with the X-ray tube and the subject interposed therebetween, and that is irradiated from the X-ray tube;
A scanner that rotatably supports the X-ray tube and the X-ray detector;
A focus movement detector for detecting an X-ray irradiation position moved by thermal expansion of the anode of the X-ray tube or tilt of the scanner;
A focus movement correction device for correcting the position of the X-ray tube in accordance with the detected X-ray irradiation position;
In an X-ray CT apparatus comprising: control means for controlling preliminary exposure for irradiating only the focal movement detector with X-rays;
An X-ray shielding shutter provided between the X-ray tube and the subject so as to be movable between a position for shielding X-rays irradiated from the X-ray tube and a retracted position;
Prior to the main scan, the X-ray shielding shutter is moved to the X-ray shielding position, and X-ray preliminary exposure is performed from the X-ray tube, and at the time of preliminary exposure by the focus movement detector. An X-ray CT apparatus comprising: a control device that detects an X-ray irradiation position and controls the operation of the focus movement correction device based on the detection signal. An X-ray tube that emits X-rays;
An X-ray detector arranged to face the X-ray tube and detecting X-rays emitted from the X-ray tube;
A scanner that rotatably supports the X-ray tube and the X-ray detector;
A focus movement detector for detecting an X-ray irradiation position moved by thermal expansion of the anode of the X-ray tube or tilt of the scanner;
A focus movement correction device for correcting the position of the X-ray tube in accordance with the detected X-ray irradiation position;
In an X-ray CT apparatus comprising: control means for controlling preliminary exposure for irradiating only the focal movement detector with X-rays;
The control means sets the pre-exposure execution condition to (1) a resting state of the X-ray tube for a certain period of time,
(2) The scanner tilt angle is different from the previous scan,
In either state
Wherein the preliminary exposure performed by execution condition, X-rays CT apparatus characterized by controlling so as to perform preliminary movement of Thus the X-ray tube to the focus movement correcting device just prior to the scan.   The control means, when the subject exists between the X-ray tube and the X-ray detector before the preliminary exposure, the X-rays irradiated during the preliminary exposure period, The X-ray CT apparatus according to claim 2, wherein the X-ray CT apparatus is shielded by a collimator capable of partially blocking or a shutter capable of blocking all.   The X-ray CT apparatus according to claim 3, wherein the collimator functions as the shutter. An X-ray tube that emits X-rays;
An X-ray detector that detects X-rays that are disposed opposite to each other with the X-ray tube and the subject interposed therebetween, and that is irradiated from the X-ray tube;
A scanner that rotatably supports the X-ray tube and the X-ray detector;
A focus movement detector for detecting an irradiation position of X-rays irradiated from the X-ray tube;
A collimator for reducing the X-ray,
A focus movement correction apparatus that corrects the position of the X-ray tube or the position of the collimator according to the detected irradiation position of the X-ray;
In an X-ray CT apparatus comprising: control means for controlling preliminary exposure for irradiating only the focal movement detector with X-rays;
An X-ray shielding shutter provided between the X-ray tube and the subject so as to be movable between a position for shielding X-rays irradiated from the X-ray tube and a retracted position;
Prior to the main scanning, the X-ray shielding shutter is moved to the X-ray shielding position, and X-ray preliminary exposure is performed from the X-ray tube. An X-ray CT apparatus comprising: a control device that detects an X-ray irradiation position and controls the operation of the focus movement correction device based on the detection signal.

Images