JP2007282740A - X-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus which can reduce the X-ray exposure amount, and can acquire an image of a high picture quality by optimally controlling the X-ray irradiation amount based on X-ray permeation data which are measured at an actual scanning angle and/or an actual position. <P>SOLUTION: A control signal controls the beam deflection amount of the X ray in such a manner that the X ray generated from an X-ray tube 6a is made to radiate to a section of a subject at an ahead angle and/or an ahead body axis position more than the present scanning angle and/or the present body axis position of the subject. The control signal is formed by an X-ray beam deflection controlling means 10. The beam of the X ray is deflected by an X-ray beam deflecting means 6b by the formed control signal. The deflected X-ray beam is made to radiate to the ahead angle and/or the ahead body axis position more than the present scanning angle and/or the present body axis position of the subject, and the X-ray intensity which has permeated the angle and/or the body axis position is detected by the predecessor angle and/or by a position X-ray detecting means 7b. The X-ray dosage which is made to radiate to the subject at the predecessor angle and/or the body axis position is determined by an X-ray dosage determining means 9 based on the detected value. The X-ray dosage which has been determined by the X-ray dosage determining means is controlled by an X-ray controlling means (X-ray high-voltage device) 8. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an X-ray CT apparatus, and more particularly to a technique for optimizing an X-ray dose applied to a subject and reducing the X-ray dose to the subject.

The X-ray CT apparatus irradiates a subject with a fan-shaped X-ray beam from an X-ray tube, detects X-rays transmitted through the subject with an X-ray detector disposed at a position facing the X-ray tube, The detected data is image-processed to obtain a tomographic image of the subject.
The X-ray detector is composed of hundreds of detection element groups arranged in an arc shape, and is arranged to face the X-ray tube across the subject, corresponding to the number of detector elements. A number of radially distributed X-ray passages are formed, and the X-ray tube and the detector are integrated to rotate around the subject by at least 180 degrees to detect transmitted X-rays of the subject at certain angles. .

  In this X-ray CT apparatus, in recent years, a spiral CT has been developed that continuously rotates the X-ray tube and the X-ray detector around the subject, and moves and measures the bed on which the subject is placed, Furthermore, a multi-slice X-ray CT system has been developed that can array multiple rows of X-ray detector elements in the slice direction, collect 2D X-ray data by a single X-ray exposure, and obtain multiple CT images. It has features such as “a wide range of scanning is possible in a short time” and “continuous data is obtained in the body axis direction, thereby generating a three-dimensional image”, and is widely used for diagnosis.

As described above, in the X-ray CT apparatus, high performance and high performance have progressed and the diagnostic ability has been improved to various levels. On the other hand, it is necessary to consider the reduction of the X-ray dose to the subject. This is because the thickness of the examination site of the subject, that is, the X-ray absorption rate differs depending on the angle at which the X-ray is irradiated, so the X-ray irradiation amount required varies depending on the examination site and the examination angle. The exposure X-ray dose can be reduced by appropriately controlling the X-ray irradiation amount.
For example, in the case of chest imaging, the amount of X-ray absorption is small because the lung contains a lot of air, and the appropriate X-ray irradiation amount varies depending on the X-ray irradiation angle. It is desirable to properly control the X-ray dose.

  Therefore, in order not to irradiate the subject with excessive X-rays, the current flowing between the anode and cathode of the X-ray tube during scanning (hereinafter referred to as tube current) is controlled according to the body thickness of the subject. Patent Documents 1 to 4 disclose exposure reduction techniques using an automatic exposure control function (CT Auto Exposure Control; CT-AEC function).

(1) [Patent Document 1]
Before measuring the tomographic image, a transmission image (scanogram image) for positioning to determine the measurement range is measured. According to the X-ray transmission data at each position of the measured canogram image, The tube current at each position is set, and the tube current is controlled so as to be the set tube current, thereby controlling the X-ray irradiation amount.

(2) [Patent Document 2]
The tube current at each position at the time of tomographic imaging is set and controlled using the transmission data before the scan trajectory. During tomographic measurement, the angle before the current position and the transmission before one cycle. Based on the data, a control signal (tube current control signal) for controlling the X-ray dose at each position is obtained by extrapolation and controlled.

(3) [Patent Document 3]
During tomographic measurement, the tube current in one subsequent rotation is calculated based on the transmission data from the detector row located in the front row in the direction in which the measurement proceeds, and the X-ray irradiation dose for the subsequent detector row To control.

(4) [Patent Document 4]
Using a plurality of X-ray sources, the X-ray irradiation amount from the subsequent X-ray source is controlled based on the transmission data from the X-ray source located in the forefront of the measurement direction during tomographic image measurement.
JP-A-9-108209 Japanese Patent Laid-Open No. 10-309271 JP 2002-306468 A JP 2005-73765 A

However, the technique according to the above patent document has the following problems.
In other words, in the technique using the scanogram image according to Patent Document 1, the X-ray dose corresponding to the body axis direction of the subject can be controlled from the scanogram image from only one direction, but the X-ray corresponding to the cross-sectional shape of the subject The dose control is not possible.
In addition, when a scanogram image from two directions is used, the examination time becomes longer and the exposure X-ray dose increases.
Furthermore, since no contrast agent is used in scanogram imaging for positioning, the amount of X-ray attenuation differs from that in tomographic imaging using an actual contrast agent, and optimal control for reducing the exposure X-ray dose is possible. Can not.

  In the technique for obtaining and controlling the tube current at each position during tomographic imaging by extrapolation using transmission data before the scan trajectory according to Patent Document 2, X-ray transmission at a position and angle before the current position Since the X-ray dose is controlled by the control signal based on the data, for example, when the degree of X-ray absorption of the subject suddenly changes between the abdomen and the chest, etc. Can not.

  In the case of Patent Document 3 or Patent Document 4, since the X-ray transmission data at the forefront position of the X-ray detector array or the X-ray tube is used, the X-ray irradiation amount at the foremost position cannot be optimized.

  The object of the present invention has been made in view of the above problems, and by controlling the X-ray irradiation amount optimally based on the X-ray transmission data measured at the actual position, the X-ray exposure amount can be reduced and the image quality can be improved. An object is to provide an X-ray CT apparatus capable of acquiring an image.

  The above object is achieved by detecting data transmitted through the subject at a scan angle and / or body axis position of the subject prior to the present, and controlling the X-ray irradiation dose based on the detected data. Specifically, this is achieved by the following means.

  (1) an X-ray tube that generates X-rays that irradiate the subject, an X-ray detector that sandwiches the subject and faces the X-ray tube and detects X-rays transmitted through the subject, and the X-ray detector A scanner rotator mounted with a ray tube and an X-ray detector and rotating around the subject; a predetermined angle of the scanner rotator detected by the X-ray detector; and a position of the subject in the body axis direction In the X-ray CT apparatus for reconstructing a tomographic image of the subject based on the transmitted X-ray dose transmitted through each subject, X-ray beam deflecting means for deflecting the X-ray beam generated in the X-ray tube, and X-ray beam deflection control means for controlling the deflection amount of the X-ray beam, and the subject is irradiated with the X-rays deflected by the X-ray beam deflection means to advance the current angle and / or body axis position. Preceding angle and / or position X-ray detection means for detecting X-rays transmitted through the subject at the angle and / or body axis position Timing generating means for generating timing for deflecting the X-ray beam and timing for returning the deflected X-ray beam, and the preceding angle based on the detected value detected by the preceding angle and / or position X-ray detecting means And / or X-ray dose determining means for determining the X-ray dose to be irradiated to the subject at the body axis position, and X-ray control means for controlling the X-ray dose determined by the X-ray dose determiner. .

  (2) The X-ray beam deflection control means includes an X-ray focal position detection means for detecting an X-ray focal position of the X-ray tube, and an X-ray beam deflection target for setting a target value of the deflection amount of the X-ray beam. Value setting means, and an X-ray beam deflection amount control signal generating means for controlling the X-ray focal position detected by the X-ray focal position detection means so as to become a target value set by the X-ray beam deflection target value setting means; Comprising.

  (3) The timing generation means is an extremely short time that does not affect the image quality between the angle of the current scanner rotation unit that collects the X-ray dose that has passed through the subject and the next angle, or during reconstruction measurement data collection. Between.

  (4) The X-ray irradiation amount determining means is configured to determine the X-ray at the preceding angle and / or preceding position from the ratio of the X-ray attenuation amount of the subject detected by the preceding angle and / or position X-ray detecting means. Means for obtaining a target value of irradiation dose are provided.

As described above, the X-ray generated from the X-ray tube is radiated to the subject portion at the current scan angle and / or the body axis position of the subject earlier than the body axis position. A control signal for controlling the beam deflection amount of the line is generated by the X-ray beam deflection control means, and the X-ray beam is deflected by the X-ray beam deflection means by the generated control signal. And, between the angle of the current scanner rotation unit that collects the X-ray dose transmitted through the subject by the timing generation means and the next angle or in an extremely short time without affecting the image quality during the reconstruction measurement data collection The X-ray beam is deflected and the transmitted X-ray intensity at the angle and / or body axis position ahead of the current scan angle and / or body axis position transmitted through the subject is detected by the preceding angle and / or position X-ray detection means. Based on the detected value, the X-ray dose to be irradiated to the subject at the preceding angle and / or body axis position is determined by the X-ray dose determining means, and the X dose determined by the X-ray dose determining means is determined. The subject is irradiated with X-ray control means.
As a result, data necessary for reconstruction of a tomographic image is measured with an X-ray irradiation amount based on an X-ray detection value transmitted through the subject at an actual scan angle and / or body axis position. It becomes appropriate according to the examination site and examination angle, and the exposure X-ray dose can be reduced.

  (5) The preceding angle and / or position X-ray detection means has a physical size of the X-ray detection element from an X-ray detection element of an X-ray detector that detects measurement data necessary for reconstruction of the tomographic image. The structure is also large.

  By configuring the preceding angle and / or position X-ray detection means in this way, the processing of the X-ray detection element of this detection means becomes easy, and the required physical strength of the X-ray detection element may be small. Therefore, the preceding angle and / or position X-ray detection means can be made inexpensive.

(6) Among the X-ray detection elements of the X-ray detector that detects measurement data necessary for reconstruction of the tomographic image, the X-ray detection element on which the X-rays are not incident is the preceding angle and / or position X-ray detection means And
As a result, the preceding angle and / or position X-ray detection means is not required, and an inexpensive X-ray detector can be obtained.

  According to the present invention, data necessary for reconstruction of a tomographic image is measured with an X-ray irradiation amount based on an X-ray detection value transmitted through a subject at an actual scan angle and / or body axis position. The irradiation amount becomes appropriate according to the examination site and examination angle, and an X-ray CT apparatus capable of reducing the exposure X-ray dose and obtaining a high-quality image can be provided.

Hereinafter, preferred embodiments of the X-ray CT apparatus of the present invention will be described in detail with reference to the accompanying drawings.
First Embodiment

FIG. 1 is a block diagram showing an overall configuration of an X-ray CT apparatus capable of spiral scanning according to the first embodiment of the present invention.
This X-ray CT apparatus irradiates a subject with X-rays, collects transmission X-ray data of the subject, and reconstructs the collected X-ray data to obtain a tomographic image. Based on the X-ray data collected by the scanner gantry 1 that irradiates X-rays and collects X-ray data transmitted through the subject, the movable bed 2 on which the subject is placed, and various operation settings And an operation console 3 for reconstructing and displaying an X-ray tomogram.

  The scanner gantry 1 is composed of a scanner fixing part (not shown) and a scanner rotating part 4, and an opening 5 is provided at the center of the scanner gantry 1, and the scanner gantry 1 has a front face on the front side of the scanner gantry 1. A bed 3 is arranged.

The scanner rotation unit 4 includes an X-ray source 6 that generates X-rays to be irradiated on the subject, an X-ray detector 7 that collects X-ray transmission data transmitted through the subject, and the X-ray source 6 X-ray high-voltage generator 8 that supplies power for generating X-rays, X-ray dose determining unit 9 that determines the dose of X-rays irradiated to the subject, and radiation from the X-ray source 6 And an X-ray beam deflection control means 10 for controlling the deflection of the X-ray beam direction. The scanner control device 11 generates a scanner control signal corresponding to the operation command from the operation console 3, and this control signal The rotation unit 12 rotates the scanner rotation unit 4 based on the rotation of the scanner rotation unit 4 at a predetermined rotation speed, and a commercial AC power supply (not shown) is connected via a power supply mechanism including a slip ring and a brush (not shown). To supply power to the X-ray high voltage apparatus 8.
Then, the fixed scanner gantry body performs a continuous rotation scan and simultaneously moves the bed 2 in the body axis direction of the subject, thereby causing the X-ray source 6 to perform a spiral motion relative to the subject. To do.
In FIG. 1, illustration of a collimator having a slit for limiting the X-ray irradiation range and a bed control unit for controlling the bed 2 is omitted.

  The operation console 3 is provided with an input device such as a keyboard and a mouse (not shown), and a display device for displaying various information such as patient information and imaging conditions, a tomographic image taken, and the like. Accommodates an image processing device (not shown) and a control device (not shown) for controlling the entire system.

  The X-ray source 6 includes an X-ray tube that generates X-rays, which will be described later, and an X-ray deflecting unit that deflects the beam direction of the X-rays generated by the X-ray tube. As disclosed in Japanese Patent Laid-Open No. 2000-48748, the X-ray beam direction is controlled to be deflected so that the electron beam of the X-ray tube corresponds to a target position where the position of the focal point on the anode changes as a function of time. Is.

FIG. 2 shows how the X-ray beam generated from the X-ray tube is deflected by the X-ray deflecting means.
In FIG. 2, the X-ray source 6 comprises an X-ray tube 6a and an X-ray deflecting means 6b, and the X-ray emitted from the X-ray source 6 is collimated by a collimator 13 and irradiated to a subject not shown in the figure. X-rays that have passed through the subject enter the X-ray detector 7.

  FIG. 2 (a) shows a normal X-ray beam in which the X-ray beam direction radiated from the X-ray tube 6a is not deflected by the X-ray deflecting means 6b. This X-ray has a slice thickness set by the collimator 13. After passing through the subject with a limited width, the X-ray detector 7 enters the portion 7a for detecting measurement data necessary for reconstruction of the tomographic image.

  Fig. 2 (b) shows the X-ray beam when detecting X-ray transmission data for determining the X-ray irradiation dose ahead of the current position. This X-ray is the slice thickness set by the collimator 13. The X-ray beam irradiation direction is deflected in the direction ahead of the current position by the X-ray deflecting means 6b (the X-ray beam deflecting means in the claims) It also enters the portion 7b (preceding angle and / or position X-ray detection means in claims) for detecting X-ray transmission data for determining the X-ray irradiation amount at a position ahead of the position.

  The X-ray tube 6a generates X-rays by colliding an electron beam generated from a cathode against an anode target, and the trajectory of the electron beam is arbitrarily set by a coil (not shown) inside the X-ray deflecting means 6b. The direction of the X-ray beam is deflected by changing the position where the electron beam collides with the anode.

This X-ray deflecting means 6b has an amount and timing according to a target value (for example, pulse width, analog value) input from the scanner control device 11, as disclosed in the above Japanese Patent Laid-Open No. 2000-48748. By applying a voltage to a coil (not shown) inside the X-ray deflecting means 6b and controlling the trajectory of the electron beam with the X-ray beam deflection control means 10 (X-ray beam deflection control means in the claims), The X-ray beam can be deflected in any direction at any timing.
Accordingly, the deflection control in the X-ray beam direction can be performed by the X-ray deflecting means 6b following the target value corresponding to the scan trajectory obtained from the parameters such as the scan speed and the bed feed amount notified from the scanner control device 11. .

Also, as shown in FIG. 2, by making the inside of the anode of the X-ray tube 6a protrude, the X-ray beam can easily reach a position exceeding the collimator 13 as shown in FIG. Become.
By making the protrusion on the inside of the anode an angle as a spherical curved surface, the range of X-ray irradiation can be made wider, so that X-ray transmission data at further positions can be detected. Become.

FIG. 3 is a view of the detector row of the X-ray detector 7 as viewed from the X-ray irradiation direction. The portion 7a for detecting measurement data necessary for reconstruction of the tomographic image in the X-ray detector 7 is, for example, a channel 1000 detection elements are arranged in the direction, and 64 detection elements are arranged in the slice direction (Z-axis direction).
Then, as a part 7b for detecting the X-ray transmission data for determining the X-ray irradiation amount at the position ahead of the current position in the scanning direction, for example, 200 channels in the scanner rotation direction, the slice direction (Z-axis direction) ) 64 additional detector elements are arranged. That is, the detection element of the X-ray detector 7 is 1.2 times in the channel direction and 2 times in the Z-axis direction.

In this way, by detecting the preceding data with the detection element added in the column direction (Z-axis direction) and adding the detection element in the channel direction as well, the preceding data can be obtained even in the case of a subject larger than the normal channel width. Can be collected.
The number of detection elements to be added in the row direction is determined so that it can correspond to the entire range of the feed amount (spiral pitch) of the bed 2 per rotation of the scanner rotation unit 4 which is the specification of the X-ray CT apparatus, and the channel The direction may be determined by the maximum value of the lateral width (width perpendicular to the body axis direction) of the subject.

  In the first embodiment configured as described above, when an operator inputs measurement conditions such as a scan speed, an X-ray condition, and a measurement range from an input device (not shown) of the operation console 3, the measurement conditions are set for the entire system. Is transmitted from a control device (not shown) to a scanner control device 11 via a signal transmission means (not shown) using a slip ring and a brush, and the set scan speed from the scanner control device 11 to the scanner rotation driving unit 12 A command for rotating the scanner rotating unit 4 is output and a command for irradiating X-rays from the X-ray tube 6a of the X-ray source 6 is output.

The scanner control device 11 continuously rotates the scanner rotating unit 4 at a set scanning speed and moves the bed 2 relative to the X-ray source 6 in the body axis direction of the subject so as to be in a set measurement range. Move.
Then, a DC high voltage (tube voltage) applied between the anode and the cathode of the X-ray tube 6a in the X-ray high voltage device 8 so as to satisfy the set X-ray conditions, and a current (tube current) flowing between the anode and the cathode Is controlled to irradiate the subject with the required X-rays from the X-ray tube 6a to perform a helical scan.

  Thereby, the X-ray source 6 and the X-ray detector 7 scan while rotating around the subject, and the X-ray detector 7 detects the X-ray transmission data of the subject at every predetermined rotation angle. The detected X-ray transmission data is transmitted to an image processing device (not shown) built in the operation console 3 via a signal transmission means (not shown) using a slip ring and a brush, and various image processing is performed by the image processing device. The tomographic image of the subject is displayed on the display device (not shown) of the console console 3 after completion of imaging.

  In the case where a tomographic image of a subject is obtained by scanning as described above, the present invention is directed to reducing the exposure X-ray dose resulting from the difference in the amount of X-ray transmission due to changes in the shape of the subject and the examination site. The operation is controlled so as to optimize the X-ray dose in accordance with the shape of the specimen and the examination site, and the operation thereof will be described below with reference to the flowchart of FIG.

(1) Step S11
It is determined whether or not to acquire X-ray transmission data at a position ahead of the current position, and if acquisition of X-ray transmission data at the previous position is necessary, the processing at Steps S21 to S23 is performed to process the X at the previous position. Get line transmission data.

(2) Step S21
The X-ray deflecting means 6b deflects the X-ray beam direction radiated from the X-ray tube 6a toward the front position.
As shown in FIG. 5, the forward position may be a position several degrees ahead on the measurement trajectory that advances in a spiral shape, or as shown in FIG. Alternatively, it may be a half cycle ahead, further ahead, or a plurality of these positions.

(3) Step S22
The subject is irradiated with the X-rays deflected in step S21, and X-ray transmission data for determining the X-ray irradiation amount at a position ahead of the current position is acquired by the detection unit 7b of the X-ray detector 7.
(4) Step S23
The X-ray deflecting means 6b returns the X-ray beam deflection direction in step S21 to the normal X-ray beam deflection direction.

(5) Step S12
Based on the X-ray transmission data acquired in step S22, an appropriate X-ray dose at the X-ray irradiation position is determined by the X-ray dose determining unit 9 (X-ray dose determining means in claims) The tube current is controlled by the X-ray high voltage apparatus 8 (X-ray control means in claims) so that the determined X-ray irradiation amount is obtained.

(6) Step S13
Normal measurement data for obtaining a tomographic image is acquired.
(7) Step S14
It is determined whether or not the measurement has been completed up to the set measurement range. If the measurement has not been completed, the process returns to step S11 to continue the measurement. If the measurement has been completed, the measurement is terminated.

  The timing for acquiring X-ray transmission data for determining the X-ray irradiation amount at the position ahead of the current position by the processing of steps S21 to S23 (the timing generation means in the claims) is 1 by the scanner rotation unit 4. For example, if the measurement data is collected 1000 times (1000 views) while changing the rotation angle during the rotation of 360 degrees, which is the cycle, the time between the data acquisition (between the views), that is, the fault X-ray transmission data collection at the next angle is started from the completion of X-ray transmission data collection at the angle detected by the portion 7a of the X-ray detector 7 that detects measurement data necessary for image reconstruction. Or the preceding data collection process (steps S21 to S23) may be performed for an extremely short time that does not affect the image quality during normal measurement data collection. Get data The timing is not limited.

  The scanner rotation unit 4 is controlled so as to have a scan speed set by the scanner rotation drive unit 12, and a rotary not shown which is an angle detection unit of the scanner rotation unit 4 built in the scanner rotation drive unit 12 The angle of the scanner rotating unit 4 is detected by counting pulses emitted from the encoder according to the scanner rotation. As a result, the scanner control device 11 causes the X-ray beam deflection control means 10 to send the X-ray beam deflection target value setting means (not shown) to the X-ray beam deflection target value setting means according to the angular position of the scanner rotating unit 4. The X-ray beam deflection target value set by the target value setting means) is input, and the X-ray beam deflection control means 10 controls the X-ray beam deflection direction of the X-ray deflection means 6b so that this target value is obtained. To do.

  The X-ray deflection means 6b controls the deflection direction of the X-ray beam by detecting the current position of the X-ray focal point as disclosed in Japanese Patent Laid-Open No. 2000-48748 (the X-ray focal point of the claims). Position detection means), and generates a control signal for controlling the detection value to coincide with the target value (X-ray beam deflection amount control signal generation means in the claims), and the X-ray beam deflection control means by this control signal 10 controls the current supplied to the deflection coil of the X-ray deflection means 6b.

Although detection of the current position of the X-ray focal point is omitted, it is provided in front of the collimator 13 or from a position signal corresponding to the current position of the X-ray focal point using a signal detected by the X-ray detector 7. Ask.
Further, since the target value of the X-ray beam deflection is determined in advance from the scanner rotation speed set by the operation console 3 and the bed feed amount per scanner rotation (hereinafter referred to as table pitch), In the case of FIG. 6 and FIG. 10 described later, the distance is the same as the table pitch, and in the case of FIG. 5, it is limited to the response time of the X-ray dose control, but is several degrees ahead on the spiral trajectory.

  The measurement data capture timing is generated based on the angle detection signal of the scanner rotation unit 4 detected by the rotary encoder. For example, the output pulse of the rotary encoder of 10000 (pulse / rotation) is divided by 10 and 1000 (times / rotation). ) Is generated. Therefore, the X-ray beam deflection target value is input to the X-ray beam deflection control means 10 in accordance with the timing of taking the measurement data from the scanner control device 11, and the X-ray transmission data at the preceding position is acquired at the timing. In synchronism, the X-ray deflecting means 6b deflects the X-ray beam to the target value of the X-ray beam deflection.

In order to obtain an appropriate X-ray dose in step S12, for example, assuming that the shape of the subject is an ellipse, the X-ray source 6 is vertically and horizontally (scanner rotation angle 0 degrees, 90 degrees, 180 degrees, 270 degrees), the previous X-ray transmission data is collected, and the sinusoidal amplitude is obtained from the ratio of the X-ray attenuation amount between the vertical position and the horizontal position, and the target value of the X-ray irradiation amount is obtained. The value is changed in a sine wave shape.
The sinusoidal target X-ray irradiation amount is generated by the scanner control device 11, and this target value is input to the X-ray high voltage device 8 to control the X-ray irradiation amount so as to be the target value. .
Since the sinusoidal amplitude corresponds to the shape of the examination site of the subject, the X-ray irradiation dose is appropriately controlled corresponding to the examination site.

In addition, if the X-ray transmission data collection of the subject is not elliptical if the previous X-ray transmission data collection is not limited to the vertical and horizontal positions, but is performed more finely, for example, for each view as described above. X-ray irradiation dose can be controlled.
In this case, since the general rotation / rotation type scan speed is about several hundreds (msec / rotation), the X-ray irradiation amount is emitted from the filament by using the X-ray transmission data one cycle ahead. Even if it depends on the amount of X-rays, the X-ray dose can be controlled following the target X-ray dose.

In addition to the X-ray dose control based only on the cross-sectional shape, X-ray dose control in the Z-axis direction is generally performed and can be applied to such X-ray dose control. is there.
Therefore, when applied to the X-ray dose control in the Z-axis direction, it is necessary to control the X-ray dose so that the variation (SD value) of the noise (SN ratio) of the X-ray transmission data is constant. Therefore, the desired SD value set on the operation console 3 is compared with the SD value of the X-ray transmission data at the previous position, and the maximum value of the X-ray irradiation amount for tomographic reconstruction can be varied by that ratio. Control.
Furthermore, the X-ray dose control in the cross-sectional direction and the X-ray dose control in the Z-axis direction may be performed simultaneously.

By adopting the first embodiment, X-ray transmission data at a position ahead of the current position is detected, and measurement data necessary for reconstruction of a tomographic image is detected based on the detected data. Since the configuration is such that the X-ray irradiation dose is controlled, the X-ray irradiation dose is appropriately controlled according to the examination site and the examination angle, and thereby the exposure X-ray dose can be reduced.
<< Second Embodiment >>

  As described in the first embodiment, in addition to the detection unit 7a that detects measurement data necessary for the reconstruction of the original tomographic image, the present invention calculates the X-ray irradiation dose at a position ahead of the current position. A detection unit 7b for detecting X-ray transmission data for determination is provided, and the X-ray irradiation dose for detecting measurement data necessary for reconstruction of tomographic images is controlled based on the data detected by the detection unit 7b. Thus, the exposure X-ray dose to the subject is reduced.

Thus, since the data detected by the detection unit 7b is for determining the X-ray irradiation dose, the density resolution is more important than the spatial resolution.
Therefore, the detection unit 7b may not be required to process the detection element minutely and with high accuracy like the detection unit 7a that detects measurement data necessary for reconstruction of a tomographic image.

FIG. 7 shows that the X-ray detection element of the detection unit 7d for determining the X-ray irradiation dose is configured with a larger physical size than the X-ray detection element of the detection unit 7c for collecting original image reconstruction data. X-ray detector.
By configuring the X-ray detector in this way, the processing of the X-ray detection element of the detection unit 7d for determining the X-ray irradiation amount becomes easy, and the required physical strength of the X-ray detection element is also achieved. Since it may be small, an X-ray detector that is less expensive than the X-ray detector of the first embodiment (FIG. 3) can be obtained.

Further, since the detection sensitivity is improved because the surface area of the detection element of the detection unit 7d is increased, the detection element of the detection unit 7d is made of an inexpensive material that is less sensitive than the detection unit 7c for collecting original data. In addition, it is possible to secure the detection sensitivity necessary for determining the X-ray irradiation amount, thereby making the X-ray detector more inexpensive.
<< Third Embodiment >>

When it is desired to obtain a high-quality tomographic image, the spiral scan trajectory may be shortened by reducing the feed amount of the bed 2 per rotation of the scanner rotation unit 4 (decreasing the table pitch).
In such a case, the X-ray beam width is limited to be narrow by the collimator 13 as shown in FIG.

  In the third embodiment of the present invention, the region not irradiated with X-rays is used for data collection for determining the X-ray irradiation amount. FIG. 8 shows an example of the arrangement of the detection elements of the X-ray detector. The detection unit 7e of the X-ray detector includes, for example, 1000 channels × 64 rows of detection elements, and includes 16 rows of detection units 7e1 out of the 64 rows.

When the table pitch is large and the X-ray beam width is wide, all of the 64 columns are used as the measurement data detection unit 7e for obtaining a tomographic image, but when the table pitch is small and the X-ray beam width is narrow , 16 out of 64 are used as a normal measurement data detection unit 7e1 for obtaining a tomographic image.
The other detector rows are used as a detector 7e2 (48 rows) for detecting X-ray transmission data for determining the X-ray irradiation dose.

As described above, the third embodiment of the present invention does not need to add a detection unit for determining the X-ray irradiation dose, so the first embodiment (FIG. 3) and the second embodiment (FIG. An X-ray detector that is even cheaper than 7) can be obtained.
<< Fourth Embodiment >>

When the size of the tomographic image to be taken is small, that is, when the FOV (Field of View) is reduced, the required detector width can be reduced.
In that case, as shown in FIG. 9, the detection unit 7h, which is the surplus in the channel direction, of the X-ray detector 7g is used as an X-ray transmission data detection unit for determining the X-ray irradiation amount.

  As described above, the example in which the present invention is applied to a general rotation / rotation type X-ray CT apparatus has been described. However, the present invention is not limited to this, and “the X-ray irradiation dose at a position ahead of the current position is determined. Based on the data detected by the X-ray detection unit that detects X-ray transmission data for determination, the X-ray irradiation amount for detecting measurement data necessary for reconstruction of tomographic images is controlled to the subject. Patent Document 4 (Japanese Patent Laid-Open No. 2005-73765) discloses a stationary / rotation type X-ray CT apparatus in which only an X-ray source rotates within a range not departing from the gist of the present invention, which is intended to reduce the exposure X-ray dose. The present invention can also be applied to a multi-tube X-ray CT apparatus such as the above, or an electron beam scanning X-ray CT apparatus using a ring-shaped target as an X-ray source.

  In addition, the gantry can be scanned with the bed fixed and the gantry moved, or both the gantry and the bed moved, and a gantry self-propelled X-ray CT device and a helical scan that scans in a spiral manner can be used. It can also be applied to normal scan X-ray CT systems.

  In the case of the normal scan, as shown in FIG. 10, the couch is not moved during measurement data collection, and the couch is sent every time one scan is performed, so the X-ray transmission data for scanning at the next slice position is sent to the destination. X-ray transmission data is collected and used to control the X-ray dose during the next slice scan.

1 is a block diagram showing an overall configuration of an X-ray CT apparatus capable of spiral scanning according to a first embodiment of the present invention. The figure which shows a mode that the beam of the X-ray generated from an X-ray tube is deflected by the X-ray deflection means. The figure which looked at the detector row | line | column of an X-ray detector from the X-ray irradiation direction. FIG. 6 is an operation explanatory diagram of the first embodiment. The figure which shows the relationship between the detection part of an X-ray detector, and a spiral orbit (relationship between the present angle and the angle of several degrees ahead). The figure which shows the relationship between the detection part of a X-ray detector, and a spiral orbit (Relationship between the present angle and the angle of the same 1 period ahead as this angle). The figure which looked at the detector row | line | column of the X-ray detector in 2nd Embodiment from the X-ray irradiation direction. The figure which looked at the detector row | line | column of the X-ray detector in 3rd Embodiment from the X-ray irradiation direction. The figure which looked at the detector row | line | column of the X-ray detector in 4th Embodiment from the X-ray irradiation direction. The figure which shows the relationship between the detection part of a X-ray detector and a scanning orbit in normal scanning.

Explanation of symbols

  1 Scanner gantry, 2 bed, 3 operation console, 4 scanner rotation part, 5 aperture, 6 X-ray source, 6a X-ray tube, 6b X-ray deflection means, 7 X-ray detector, 7a For reconstruction of tomogram Necessary data detection unit, 7b X-ray transmission data detection unit to determine the X-ray irradiation dose ahead of the current position, 8 X-ray high voltage generator, 9 X-ray irradiation dose determination unit, 10 X Line beam deflection control means, 11 scanner control device, 12 scanner rotation drive unit, 13 collimator

Claims (4)

  An X-ray tube for generating X-rays to irradiate the subject, an X-ray detector for detecting X-rays that are transmitted through the subject while being placed opposite to the X-ray tube with the subject interposed therebetween, and the X-ray tube, A scanner rotator mounted with an X-ray detector and rotating around the subject; a subject at a predetermined angle of the scanner rotator detected by the X-ray detector and a position of the subject in the body axis direction; In an X-ray CT apparatus for reconstructing a tomographic image of the subject based on a transmitted X-ray dose transmitted through the specimen, an X-ray beam deflecting means for deflecting an X-ray beam generated in the X-ray tube, and the X-ray X-ray beam deflection control means for controlling the deflection amount of the beam, X-rays deflected by the X-ray beam deflection means are applied to the subject, and an angle ahead of the current angle and / or body axis position and // A leading angle for detecting X-rays transmitted through the subject at the body axis position And / or position X-ray detection means, timing generation means for generating timing for deflecting the X-ray beam and timing for returning the deflected X-ray beam, and the preceding angle and / or position X-ray detection means Based on the detected value, the X-ray dose determining means for determining the X-ray dose irradiated to the subject at the preceding angle and / or body axis position, and the X-ray dose determined by the X-ray dose determining means are controlled. X-ray CT apparatus comprising:   The X-ray beam deflection control means includes an X-ray focal position detection means for detecting an X-ray focal position of the X-ray tube, and an X-ray beam deflection target value setting means for setting a target value of the deflection amount of the X-ray beam. And an X-ray beam deflection amount control signal generating means for controlling the X-ray focal position detected by the X-ray focal position detecting means to be a target value set by the X-ray beam deflection target value setting means. The X-ray CT apparatus according to claim 1.   The timing generation unit is between the angle of the current scanner rotation unit that collects the X-ray dose transmitted through the subject and the next angle, or for an extremely short time that does not affect the image quality during the reconstruction measurement data collection. The X-ray CT apparatus according to claim 1 or 2, wherein   The X-ray irradiation amount determining means determines the X-ray irradiation amount at the preceding angle and / or preceding position from the ratio of the X-ray attenuation amount of the subject detected by the preceding angle and / or position X-ray detecting means. 4. The X-ray CT apparatus according to claim 1, further comprising means for obtaining a target value.

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