JP3466678B2 - X-ray CT scanner - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT scanner for taking a tomographic image of a subject, and more particularly to a technique for making the X-ray dose constant after passing through the subject when performing a helical scan.

[0002]

2. Description of the Related Art In recent years, X-ray CT scanners are widely used as medical diagnostic apparatuses are being developed. X-ray C
The T-scanner causes a subject to enter an imaging region where an X-ray tube and an X-ray detector are arranged to face each other, irradiates an X-ray beam from the X-ray tube, and detects X-rays after passing through the subject. Collect in a container. Then, this operation is repeated while changing the X-ray irradiation angle to obtain measurement data from each direction, convolve each measurement data, and perform processing such as backprojection to reconstruct the CT image.

In such an X-ray CT scanner, a so-called helical scan has recently been adopted, which draws a spiral trajectory by rotating an X-ray tube and an X-ray detector and moving an object in a body axis direction. There is. Conventionally, when a helical scan is executed, the scan conditions (tube voltage, tube current, etc.) during one scan are the same. That is, control for changing the tube voltage or the tube current is not performed during the scan. Therefore, the dose may be excessive or insufficient depending on the X-ray irradiation angle and the irradiation site.

For example, as shown in FIG. 8, when performing a helical scan on the chest of the subject 101 along the trajectory indicated by reference numeral 102, the X-ray transmissivity is not always the same at the parts P1 and P2 in the body axis direction. However, it is difficult to set the optimum X-ray dose in both parts P1 and P2 under a constant tube voltage and tube current. This can be said not only in the body axis direction but also in the body radial direction.

FIG. 10 is a diagram schematically showing a cross section of the chest of the subject 101, where θ1 and θ2 indicate angular positions of the X-ray tube. Further, FIG. 9 shows the transmission X with respect to the angular position of the X-ray tube.
It is a characteristic diagram showing a dose, and as is clear from the figure, the transmitted X-ray dose is small at the angle θ1 where the subject thickness is large, and the transmitted X-ray amount is large at the angle θ2 where the subject thickness is small (dynamic range R1). That is, the dose is insufficient near the angle θ1, and the dose is excessive near the angle θ2.

[0006]

As described above, in the conventional X-ray CT scanner, each part of the subject and the X-ray CT scanner are examined.
Since the dose becomes excessive or insufficient depending on the angular position of the ray tube, there are drawbacks such as giving unnecessary exposure to the patient and degrading image quality due to insufficient dose.

The present invention has been made to solve such a conventional problem, and an object thereof is to provide an X-ray CT scanner capable of always obtaining an optimum X-ray dose during helical scanning. It is in.

In order to achieve the above object, the present invention provides a bed in which a top plate on which a subject is placed is movable in the body axis direction of the subject, and an X-ray is exposed while rotating around the subject. A gantry having an X-ray tube that emits light and an X-ray detector that detects X-rays that have passed through the subject is provided, and the subject is moved by moving the subject in the body axis direction during rotation of the X-ray source. In an X-ray CT scanner that performs a helical scan on a specimen, a data collecting unit that collects output data of the X-ray detector, the subject to be fed into the gantry, the X-ray tube, and the X-ray detection. Image capturing means for irradiating X-rays from the X-ray tube with the vessel fixed, a scano image forming means for forming a scano image on the basis of output data collected by the data collecting section, and the scano image forming means. Above A scan range setting means for setting the scan range to perform the helical scan,
Collected by the data collection unit in the scanogram imaging.
Based on the output data obtained,
The output characteristic of the X-ray source in the body axis direction is determined by the scanout image.
The X-ray examination in the body axis direction obtained by radiography
The helicopter that is almost symmetrical with the output characteristics of the output device
Scan to determine the tube current value in the scan range
A catcher down condition determining means, in accordance with the tube current value determined in the scan condition determining means, the tube current supplied to the X-ray source at the time of helical scan the set scan range in the scan range setting means And a scan control means for adjusting.

[0009]

[0010]

According to the present invention , the collection is performed in the scanning of the scanogram.
Body axis in the scan range based on the output data
The output characteristics of the X-ray source in the direction of
The output characteristics of the X-ray detector in the body axis direction obtained by
Scan at the time of helical scan so that it becomes almost symmetrical
The tube current value in the range is determined, and the tube current in the helical scan range is adjusted according to the determined tube current value. As a result, unnecessary exposure can be prevented regardless of the imaging position of the subject, and the S / N ratio of the image can be improved.

[0011]

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a perspective view showing the configuration of a general X-ray CT scanner according to the present invention. In the gantry 21 shown in the figure, an X-ray tube 1 for irradiating the subject 3 with X-rays and an X-ray detector (shown in the figure) arranged opposite to the X-ray tube 1. Is not provided). Then, while moving the subject 3 placed on the top plate 2 of the bed 22 in the body axis direction at a predetermined speed, the pair of the X-ray tube 1 and the X-ray detector is indicated by the arrow 24. If the scanning is performed while rotating with the rotation, a helical scan trajectory as shown by reference numeral 25 can be obtained, and the known helical scan method can be executed.

FIG. 3 is a block diagram showing the image processing system of the X-ray CT scanner shown in FIG. 2, in which the X-ray tube 1 and the X-ray tube are sandwiched by the subject 3 placed on the top plate 2. The detector 4 and the detector 4 are arranged to face each other. The output side of the X-ray detector 4 is connected to the data collecting unit 5, and in the data collecting unit 5,
Output data of the X-ray detector 4 that detects the transmitted X-ray dose that has passed through the subject 3 by X-ray irradiation is collected, converted into a digital value, and output. Further, a detector 14 for compensation is attached to the output side of the X-ray tube 1, and this output is connected to the data acquisition unit 5 and the system control unit 8.

The system control unit 8 receives an instruction from the central control unit 9 and outputs an operation timing signal to the X-ray control unit 6, the mechanism control unit 7 and the data acquisition unit 5, and the compensating detector 14 operates. The tube voltage and the tube current when irradiating the X-ray based on the output signal are output to the X-ray controller 6.

The mechanism controller 7 horizontally moves the top 2 while controlling the amount of movement, and outputs position information to the controller 8 of the system.

The central control unit 9 forms a slice image and a scanogram from the data collected by the data collection unit 5, and outputs a scan control signal to the system control unit 8. The display unit 10 is for displaying the obtained slice image or scan image, and the input unit 11 is for inputting input data as appropriate.

FIG. 1 is a block diagram showing the internal construction of the central control unit 9 in detail.
Scano image forming means 92 and scan range storing means 93
Scan condition determining means 94 and scan control means 9
5 and image processing means 96.

The slice image reconstructing means 91 irradiates X-rays while rotating the pair of the X-ray tube 1 and the X-ray detector 4 around the subject 3 and moving the top plate 2. A slice image is reconstructed based on the obtained helical scan data.

The scano image forming means 92 includes an X-ray tube 1 and an X-ray tube 1.
With the position of the line detector fixed, the top plate 2 is moved in the body axis direction of the subject 3, and the X-rays exposed in synchronization with this movement.
A scanogram is formed based on the transmission data of the line.

The scan range storage means 93 sets a scan range based on the formed scanogram. For example, when the chest of the subject 3 is imaged, the portion of the scanogram corresponding to the chest is set as the scan range.

The scanning condition determining means 94 includes an input unit 11
The scan condition in the helical scan photographing is determined based on the condition setting and the scanograph photographing data.
Here, the scanning conditions include the tube voltage and the tube current of the X-ray tube 1.

The scan control means 95 controls the execution of the helical scan based on the output signals of the scan condition determination means 94 and the scan range storage means 93.

The image processing means 96 takes in the slice image created by the slice image reconstructing means 91 and performs appropriate image processing.

Next, the operation of this embodiment will be described.
First, the subject 3 is placed on the top plate 2, and the subject 3 is fed into the gantry 21 shown in FIG. 2 under the control of the mechanism controller 7 to fix the X-ray tube 1 and the X-ray detector 4. X in the state
Expose the line. Then, the collected data is supplied to the scano image forming means 92, and a scano image is formed based on the amount of movement of the tabletop 2 given from the system controller 8. Then, this scanogram is displayed on the display unit 10.

Then, a characteristic curve as shown in FIG. 4, for example, is obtained from this scanogram.

FIG. 4A is a characteristic diagram showing the output value of the X-ray detector 4 with respect to the moving direction distance of the top 2, and shows the data of the chest of the subject. As is clear from the figure, the detector output is small in the central part and large in the side part. That is, it can be understood that the X-ray dose is insufficient in the central portion and the X-ray dose is excessive in the side portions.

Therefore, when the helical scan is actually executed and the slice image is photographed, if an X-ray tube output having a characteristic symmetrical to the detector output as shown in FIG. The output of the line detector 4 becomes substantially constant, and it is possible to eliminate an excessive dose and an insufficient dose. Therefore, in the scan condition determining means 94, the output of the X-ray tube 1 is shown in FIG.
The X-ray tube voltage and tube current are determined so as to obtain the characteristic curve of. Normally, the tube voltage cannot be changed during scanning, so it is set to a suitable value in advance, and the tube current is adjusted during scanning to obtain the characteristics shown in FIG. 4B.

In this way, the scan control means 95 outputs the control signals for the tube voltage and the tube current under the conditions determined by the scan condition determining means, whereby the output of the X-ray tube 1 is controlled and collected during the helical scan. X-ray detector 4
The data of is almost constant. At this time, the compensation detector 14
The data collected by the data collecting unit 5 may be corrected in advance using the X-ray dose after the output of the X-ray tube 1 collected in.

In this way, when the helical scan data is collected, the slice image reconstructing means 91 reconstructs the slice image and the image processing means converts it into an image.
0 is displayed on the screen.

In this way, in this embodiment, the X-ray detector output in the body axis direction of the subject 3 is obtained using the scano data, and this X-ray detector output is actually used when performing helical scanning. Based on the value, the output value of the X-ray tube is controlled so that the X-ray detector output becomes substantially constant. Therefore, the X-ray dose does not become excessive or insufficient depending on the imaging region of the subject, the image quality is improved, and unnecessary exposure is not given.

When the scano data with respect to the moving direction of the tabletop does not change much, that is, when the change in the characteristic diagram shown in FIG. 4A is small, the scan condition is not changed during the helical scan, and the average value is not changed. Similar effects can be obtained by irradiating a constant X-ray close to

FIG. 5 shows an X-ray CT according to the second embodiment of the present invention.
3 is a block diagram showing the configuration of a central control unit 9 of the scanner. FIG. As shown in the figure, this embodiment is different from the embodiment shown in FIG. 1 in that the storage means 97 and the average transmission dose calculating means 98 are arranged on the output side of the data collecting section 5.

The storage means 97 temporarily stores the data collected by the data collecting section 5, and the stored data is read out and the slice image reconstructing means 91, the scano image forming means 92 and the scanning condition determining means are read. It is output to 94.

The average transmitted dose calculation means 98 is the storage means 9
A part of the data stored in 7 is taken out, and this average value is calculated. For example, the X-ray detector 4 shown in FIG.
If you have a channel, take out the data collected in this central 400 channels (the shaded area in the figure),
This average value is calculated. Then, the obtained average value data is output to the system control unit 8.

Then, in the second embodiment, the X-ray output value corresponding to the angular position of the X-ray tube 1 is obtained so that the average transmission dose becomes constant. The detailed operation will be described below.

FIG. 6 is an explanatory view showing the projection angle of the X-ray, and the X-ray tube performs projection on the subject 3 while rotating by a predetermined angle in the direction of the arrow in the figure. That is, the projection angles θ _n-1 , θ _n , and θ _{n + 1}
In this order, the X-ray is irradiated to the subject 3 while the positions of the X-ray tube and the X-ray detector are sequentially changed (the bed is also moved at the same time).

Then, X detected by the compensation detector 14 at the position of the average transmission dose and the angle θ _{n-1 in} the direction of the angle θ _n-1 obtained by the average transmission dose calculation means 98 shown in FIG.
Based on dose, controls the X-ray tube controller 6 so that the average transmission X-ray dose of the system controller 8, the angle theta _n direction is equal to the average transmission X-ray amount of the angle theta _n-1 direction. That is, by using the output value of the compensation detector 14, the X-ray tube 1 is actually
Since it is possible to know the X-ray dose to be emitted by the X-ray tube, it is possible to accurately control the X-ray tube control unit 6 so that the average transmitted X-ray dose becomes equal in the angular direction.

In this way, when the helical scan is performed, the control is performed so that the average transmitted X-ray dose becomes equal in each projection direction, so that there is a problem that the X-ray dose becomes excessive or the X-ray dose becomes insufficient depending on the projection direction. Disappear.

Even if the compensating detector 14 is not used, X
Since the X-ray dose emitted by the X-ray tube 1 can be simulated by the current value given to the X-ray tube 1, similar control can be performed using this simulation result. Also,
The reason why the average transmitted X-ray dose is obtained by using the output of the central 400 channels out of the 800 channels that are all detection channels is that the side channels do not pass through the subject 3 due to the rotation of the X-ray tube 1. Since the output from the X-ray tube may be detected as it is, this detection data is not used for the average value calculation. That is, the number of channels used for calculating the average value is not limited to 400 channels in the central portion, and various changes such as 300 to 500 channels can be made according to the size of the region to be imaged.

Next, in the control method of this embodiment, when a high X-ray attenuating substance such as a metal piece exists in the subject 3, theoretically, the X-ray dose to the subject 3 is infinite. It becomes big. That is, as shown in FIG.
In the presence of the line-attenuating substance, the average transmission X-ray dose significantly decreases, and for the next projection angle θ3, excessive X-rays are emitted from the X-ray tube 1 in order to restore the reduced transmission X-ray dose. As a result, the transmission X
The dose causes a protrusion as shown by T1 at the position of the angle θ3. Therefore, in order to solve such a problem, an upper limit value is set for the X-ray dose to be exposed, and the output current of the X-ray tube control unit 6 may be controlled so as not to exceed the upper limit value.
In some cases, it may be effective to set a lower limit value and perform control as necessary.

As described above, in the second embodiment, when performing the helical scan, the X-ray dose emitted from the X-ray tube 1 is controlled so that the average transmitted X-ray dose becomes substantially constant in each projection direction. Therefore, as shown in FIG. 9 of the conventional example, the change in the transmitted X-ray dose due to the projection angle does not increase, and the problems such as excessive X-ray dose and insufficient X-ray dose are eliminated. As a result, the dynamic range of the data collection unit 5 can be reduced and the image quality can be improved.

The initial value of the tube current of the X-ray tube 1 may be a preset value, or the tube current may be determined based on the scano data when a scano image is taken. . It is also possible to perform tube current control for each projection angle and tube current control using a scanogram in the body axis direction, that is, control that combines the first embodiment and the second embodiment.

In the present embodiment, the so-called third generation X-ray CT scanner in which the X-ray tube and the X-ray detector face each other has been described, but the present invention is not limited to this and the fourth generation X-ray CT scanner is not limited thereto. It is obvious that it can be applied to a line CT scanner or the like.

[0044]

As described in detail above, according to the present invention, unnecessary exposure can be prevented regardless of the imaging position of the subject, and the S / N ratio of the image can be improved. .

[0045]

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of a first embodiment of a central control unit which is a main part of an X-ray CT scanner according to the present invention.

FIG. 2 is an explanatory diagram showing an operation of performing a helical scan using an X-ray CT scanner.

FIG. 3 is a block diagram showing a control system of an X-ray CT scanner.

FIG. 4 is a characteristic diagram showing an X-ray detector output distribution and an X-ray tube output with respect to a moving direction of a tabletop.

FIG. 5 is a block diagram showing a configuration of a central control unit according to a second embodiment of the present invention.

FIG. 6 is an explanatory diagram showing a situation in which the projection direction is sequentially switched for the subject.

FIG. 7 shows that X-ray tube output can be set by setting an upper limit value.
It is explanatory drawing which shows the condition which prevents a rapid change of a dose.

FIG. 8 is an explanatory diagram showing a helical scan trajectory.

FIG. 9 is a characteristic diagram showing changes in the transmitted X-ray dose with respect to the angular position (projection angle) of the X-ray tube.

FIG. 10 is an explanatory diagram showing a projection angle with respect to a subject.

[Explanation of symbols]

1 X-ray tube 2 Top plate 3 subject 4 X-ray detector 5 Data collection department 8 System control unit 9 Central control unit 14 Compensation detector 21 Gantry 91 Slice image reconstruction means 92 Scano image forming means 93 scan range storage means 94 Scan condition determining means 95 Scan control means 97 storage means 98 Average Transmission Dose Calculation Means

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-2-224646 (JP, A) JP-A-1-293844 (JP, A) JP-A-5-192323 (JP, A) JP-A-4- 297241 (JP, A) JP 3-103229 (JP, A) JP 2-121635 (JP, A) JP 61-52860 (JP, A) JP 4-150839 (JP, A) Actual Kaihei 6-38915 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) A61B 6/00-6/14

Claims (5)

(57) [Claims] 1. A bed in which a top plate on which a subject is placed is movable in the body axis direction of the subject, an X-ray tube for irradiating X-rays while rotating around the subject, and the subject. A gantry having an X-ray detector that detects X-rays that have passed through the sample is provided, and a helical scan is performed on the sample by moving the sample in the body axis direction while the X-ray source is rotating. In an X-ray CT scanner, a data collecting unit that collects output data of the X-ray detector, the subject to be sent into the gantry, and the X-ray CT
A scanogram for forming a scanogram on the basis of output data collected by the data collecting section in scanogram imaging in which X-rays are radiated from the X-ray tube with the X-ray detector and the X-ray detector fixed. and forming means, on the scanogram, the scanning range setting means for setting the scan range to perform the helical scan, the collection of by the data acquisition unit in the scanogram imaging
Based on the output data obtained,
The output characteristic of the X-ray source in the body axis direction is determined by the scanout image.
The X-ray examination in the body axis direction obtained by radiography
The helicopter that is almost symmetrical with the output characteristics of the output device
Scan to determine the tube current value in the scan range
A catcher down condition determining means, in accordance with the tube current value determined in the scan condition determining means, the tube current supplied to the X-ray source at the time of helical scan the set scan range in the scan range setting means An X-ray CT scanner comprising a scan control unit for adjusting. 2. The scan condition determining means does not change the tube current value in a range in which the output data collected by the data acquisition unit by the scanogram imaging has a small change in the body axis direction. X-ray CT according to claim 1.
Scanner. 3. The scanning condition determining means, based on the output value of the X-ray detector at the angular position of the exposed X-ray tube before the angular position of the unexposed X-ray tube. The X-ray CT scanner according to claim 1 or 2, wherein a tube current value at an angular position of the unexposed X-ray tube is determined. 4. The scanning condition determining means, based on the output data collected in the scanogram imaging and the output value of the X-ray detector at the angular position of the exposed X-ray tube, the unexposed area. 4. The X-ray CT scanner according to claim 3, wherein a tube current value at an angular position of the X-ray tube for irradiation is determined. 5. The X according to claim 4, wherein the scan condition determining means determines an initial value of a tube current of the X-ray tube based on output data collected in the scanography. Line CT scanner.