X-ray ct scanner

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention relates to an X-ray CT scanner that captures a tomographic image of a subject, and particularly relates to a technique for making the X-ray dose after permeation of a subject constant when performing a helical scan.

【0002】

[Conventional technology]

In recent years, with the progress of development of medical diagnostic equipment, X-ray CT scanners are often used. The X-ray CT scanner invades the subject into the imaging area where the X-ray tube and the X-ray detector are arranged facing each other, irradiates the X-ray beam from the X-ray tube, and X-rays the X-ray after the subject has passed. Collect with a line detector. Then, this operation is repeated while changing the X-ray irradiation angle to obtain measurement data from each direction, convoluted into each measurement data, and processing such as deconvolution is added to reconstruct the CT image.

【0003】

In such X-ray CT scanners, so-called helical scans, in which a spiral orbit is drawn by rotating an X-ray tube and an X-ray detector and moving a subject in the body axis direction, are often adopted these days. Conventionally, when a helical scan is executed, the scan conditions (tube voltage, tube current, etc.) during one scan execution are the same. That is, there is no control to change the tube voltage or tube current during scanning. Therefore, the dose may be excessive or insufficient depending on the X-ray irradiation angle and the irradiation site.

【0004】

For example, as shown in FIG. 8, when performing a helical scan on the chest of the subject 101 in the orbit shown by reference numeral 102, the X-ray transmittances are not always the same in the body axial parts P1 and P2, and both. It is difficult to obtain the optimum X-ray dose at the sites P1 and P2 under constant tube voltage and tube current. The same can be said not only in the body axis direction but also in the body diameter direction.

【0005】

FIG. 10 is a diagram schematically showing a chest cross section of the subject 101, and θ1 and θ2 show the angular positions of the X-ray tubes. Further, FIG. 9 is a characteristic diagram showing the transmitted X-ray dose with respect to the angular position of the X-ray tube. 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 angle at which the subject thickness is small. At θ2, the transmitted X-ray dose is large (dynamic range R1). That is, the dose is insufficient in the vicinity of the angle θ1, and the dose is excessive in the vicinity of the angle θ2.

【0006】

[Problems to be Solved by the Invention]

In this way, in the conventional X-ray CT scanner, the dose becomes excessive or insufficient depending on each part of the subject and the angular position of the X-ray tube, so that the patient may be exposed to unnecessary radiation. There are drawbacks such as deterioration of image quality due to insufficient dose.

【0007】

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

【0008】

[Means for solving problems]

In order to achieve the above object, the first invention of the present application has X-ray tubes arranged relative to each other, rotates the X-ray tube and the X-ray detector, and axes the subject on the bed. In an X-ray CT scanner capable of performing a helical scan by exposing X-rays while moving in a direction, a means for performing a scanno-photograph of the subject and a helical scan are performed based on the captured scanno-photographed data. It is characterized by having a scan condition determining means for determining the optimum scan condition at the time of the operation and a scan control means for controlling the execution of the helical scan according to the scan condition.

【0009】

Further, the second invention of the present application has an X-ray tube and an X-ray tube detector arranged so as to face each other, and the X-ray tube and the X-ray detector are rotated and a subject on a bed is placed. In an X-ray CT scanner capable of helical scanning by exposing X-rays while moving in the body axis direction, when performing helical scanning, the X-ray detector is located at one angle position of the X-ray tube. An arithmetic means for calculating the average transmitted X-ray dose obtained by averaging the X-rays detected by at least some elements at each angle position of the X-ray tube and the average transmitted X-ray dose obtained by the arithmetic means become equal to a certain value. As described above, it is characterized by having a control means for controlling the current applied to the X-ray tube.

【0010】

[Action]

In the first invention of the present application configured as described above, the X-ray detector output value in the axial direction of the subject is obtained by performing scanno imaging in advance before performing the helical scan of the subject imaging unit. Here, scanning photography is to expose X-rays from an X-ray tube without rotating the X-ray tube, and to collect X-ray transmission data while moving the bed and sliding the subject. .. Then, when performing a helical scan, scan conditions such as an X-ray tube current are controlled based on this output value. For example, the X-ray tube current is increased at a position where the detector output is small, and conversely, the X-ray tube current is decreased at a position where the detector output is large, so that the X-ray detector output is controlled to be substantially constant. To do. As a result, unnecessary exposure can be prevented and the S / N ratio of the image can be prevented.

【0011】

Further, in the second invention of the present application, the X-ray tube output for each projection direction is controlled so that the average of the detected values collected in each channel of the X-ray detector becomes substantially constant. Therefore, the dynamic range in the data collection unit can be reduced, and the S / N ratio of the image can be improved.

【0012】

[Example]

Hereinafter, examples of the present invention will be described 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 exposing X-rays to the subject 3 and an X-ray detector arranged opposite to the X-ray tube 1 (shown in the figure). Not) is arranged. Then, while moving the subject 3 placed on the top plate 2 of the sleeper 22 in the body axis direction at a predetermined speed, the pair of the X-ray tube 1 and the X-ray detector is shown along the arrow indicated by reference numeral 24. If the scan is performed while rotating the X-ray tube, a helical scan trajectory as shown by reference numeral 25 can be obtained, and a well-known technique, the helical scan method, can be executed.

【0013】

FIG. 3 is a block diagram showing an image processing system of the X-ray CT scanner shown in FIG. 2, and the X-ray tube 1 and the X-ray detector 4 sandwich the subject 3 placed on the top plate 2. And are arranged relative to each other. The output side of the X-ray detector 4 is connected to the data collection unit 5, and the data collection unit 5 of the X-ray detector 4 detects the transmitted X-ray dose transmitted through the subject 3 by exposure to X-rays. Output data is collected, converted to digital values, and output. Further, a compensating detector 14 is attached to the output side of the X-ray tube 1, and this output is connected to the data collection unit 5 and the system control unit 8.

【0014】

The system control unit 8 receives a command 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 collection unit 5, and also as an output signal of the compensation detector 14. Based on this, the tube voltage and tube current when exposing X-rays are output to the X-ray control unit 6.

【0015】

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

【0016】

The central control unit 9 forms a slice image and a scanno image from the data collected by the data collection unit 5, and outputs a scan control signal to the system control unit 8. Further, the display unit 10 displays the obtained slice image or scanno image, and the input unit 11 inputs input data as appropriate.

【0017】

FIG. 1 is a block diagram showing the internal configuration of the central control unit 9 in detail. The slice image reconstructing means 91, the scanno image forming means 92, the scan range storage means 93, the scan condition determining means 94, and the scan. It is composed of a control means 95 and an image processing means 96.

【0018】

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

【0019】

The scanno image forming means 92 moves the top plate 2 in the body axis direction of the subject 3 while fixing the positions of the X-ray tube 1 and the X-ray detector, and X is exposed in synchronization with this movement. A scanno image is formed based on the line transmission data.

【0020】

The scan range storage means 93 sets the scan range based on the formed scanno image. For example, when photographing the chest of the subject 3, the part of the scanno image corresponding to the chest is set as the scan range.

【0021】

The scan condition determining means 94 determines the scan condition in the helical scan imaging based on the condition setting from the input unit 11 and the scanno image imaging data. Here, as scan conditions, the tube voltage and tube current of the X-ray tube 1 can be mentioned.

【0022】

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.

【0023】

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

【0024】

Next, the operation of this embodiment will be described. First, the subject 3 was placed on the top plate 2, and the subject 3 was sent into the gantry 21 shown in FIG. 2 under the control of the mechanism control unit 7, and the X-ray tube 1 and the X-ray detector 4 were fixed. Expose X-rays in the state. Then, the collected data is supplied to the scanno image forming means 92, and the scanno image is formed based on the amount of movement of the top plate 2 given by the system control unit 8. After that, this scanno image is displayed on the display unit 10.

【0025】

Then, a characteristic curve as shown in FIG. 4, for example, can be obtained from this scano image.

【0026】

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 plate 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 dose is insufficient in the central part and the X dose is excessive in the side part.

【0027】

Therefore, when actually performing a helical scan and taking a slice image, if an X-ray tube output having characteristics symmetrical to the detector output as shown in Fig. 4 (b) is given, the X-ray detector will be detected. The output of 4 becomes almost constant, and it is possible to eliminate excessive dose and insufficient dose. Therefore, the scan condition determining means 94 determines the X-ray tube voltage and the tube current so that the output of the X-ray tube 1 has the characteristic curve of FIG. 4 (b). Normally, the tube voltage cannot be changed during scanning, so set it to an appropriate value in advance and adjust the tube current during scanning to obtain the characteristics shown in Fig. 4 (b).

【0028】

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

【0029】

When the helical scan data is collected in this way, the slice image is reconstructed by the slice image reconstructing means 91, imaged by the image processing means, and then displayed on the screen on the display unit 10.

【0030】

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

【0031】

If the scanno data does not change much with respect to the moving direction of the top plate, that is, if the change in the characteristic diagram shown in Fig. 4 (a) is small, the scan conditions are not changed during the helical scan and are constant close to the average value. The same effect can be obtained by exposing the X-rays of.

【0032】

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

【0033】

The storage means 97 temporarily stores the data collected by the data collection unit 5, and the stored data is read out and output to the slice image reconstruction means 91, the scanno image forming means 92, and the scan condition determination means 94. Will be done.

【0034】

The average transmitted dose calculation means 98 takes out a part of the data stored in the storage means 97 and calculates the average value. For example, when the X-ray detector 4 shown in FIG. 3 has 800 channels, the data collected in the 400 channels in the center (diagonal part in the figure) is taken out and the average value is calculated. Then, the obtained average value data is output to the system control unit 8.

【0035】

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 transmitted dose becomes constant. The detailed operation will be described below.

【0036】

FIG. 6 is an explanatory diagram showing the projection angle of X-rays, and the X-ray tube is projected to the subject 3 while rotating in the direction of the arrow in the figure by a predetermined angle. That is, the projection angle θ_n-1, θ_n, θ_{n + 1}X-rays are exposed to subject 3 while the positions of the X-ray tube and the X-ray detector are changed in this order (the sleeper also moves at the same time).

【0037】

Then, the angle θ obtained by the average transmitted dose calculation means 98 shown in FIG._n-1Average permeation dose in direction and angle θ_n-1Based on the X dose detected by the compensating detector 14 at the position of, the system control unit 8 has an angle θ._nThe average transmitted X-ray dose in the direction is the angle θ_n-1The X-ray tube control unit 6 is controlled so as to be equal to the average transmitted X-ray dose in the direction. In other words, since the X-ray dose actually exposed by the X-ray tube 1 can be known by using the output value of the compensation detector 14, the X-ray tube can be accurately transmitted so that the average transmitted X-ray dose becomes equal in the angular direction. The control unit 6 can be controlled.

【0038】

In this way, when performing the helical scan, control is performed so that the average transmitted X-ray dose is equal in each projection direction, so that there is no problem that the X-ray dose becomes excessive or the X-ray dose becomes insufficient depending on the projection direction.

【0039】

Further, 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 without using the compensating detector 14, the same control can be performed using this simulation result. It is possible. The reason why the average transmitted X-ray dose is obtained by using the output of 400 channels in the center of the 800 channels that are all detection channels is that the side channels permeate the subject 3 due to the rotation of the X-ray tube 1. This is because the output from the X-ray tube may be detected as it is without using this detection data 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 can be made, for example, 300 to 500 channels, depending on the size of the imaging target portion.

【0040】

Next, in the control method of this embodiment, when a high X-ray attenuating substance such as a metal piece is present in the subject 3, the X-ray dose to be exposed to the subject 3 is theoretically infinite. .. That is, as shown in FIG. 7, when a high X-ray attenuating substance is present at the position of the angle θ2, the average transmitted X-ray dose is significantly reduced, and the reduced transmitted X-ray dose is to be restored for the next projection angle θ3. Excessive X-rays will be exposed from the X-ray tube 1, and as a result, the transmitted X-ray dose will have a protrusion as shown in T1 at the position of the angle θ3. Therefore, in order to solve such a problem, an upper limit value may be 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 value. It may also be effective to set a lower limit and control it if necessary.

【0041】

In this way, 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 is substantially constant in each projection direction. As shown in FIG. 9 of the conventional example, the change in the transmitted X-ray dose depending on the projection angle does not become large, and problems such as an excessive X-ray dose or an insufficient X-ray dose are solved. As a result, the dynamic range of the data collection unit 5 can be reduced, and the image quality can be improved.

【0042】

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

【0043】

Further, in the present embodiment, the so-called third-generation X-ray CT scanner in which the X-ray tube and the X-ray detector rotate in opposition to 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 self-evident that it can also be applied to.

【0044】

[Effect of the invention]

As described above, according to the first invention of the present application, a scanno image of the imaging region of the subject is photographed in advance, X-ray detector data in the body axis direction is collected, and a helical scan is performed based on the scanno image. Controls the scanning conditions at the time. Therefore, the problem that the X dose is excessive or insufficient in the body axis direction of the subject is solved, unnecessary exposure can be prevented, and the S / N ratio of the image is improved.

【0045】

Further, according to the second invention of the present application, since the average transmitted X-ray dose in the X-ray detector is controlled to be constant for each angular position of the X-ray tube with respect to the subject, X is X in each projection direction of the X-ray. The dose will not be excessive or insufficient, unnecessary exposure can be prevented, and the S / N ratio of the image will be improved. Furthermore, if the upper limit value and the lower limit value are set for the current value of the X-ray tube, the X-ray output will not change drastically even in an abnormal state.

[Simple explanation of drawings]

[Figure 1]

It is a block diagram which shows the structure of 1st Example of the central control part which is a main part of the X-ray CT scanner which concerns on this invention.

[Figure 2]

It is explanatory drawing which shows the operation of performing a helical scan using an X-ray CT scanner.

[Fig. 3]

It is a block diagram which shows the control system of an X-ray CT scanner.

[Fig. 4]

It is a characteristic diagram which shows the distribution of the X-ray detector output and the X-ray tube output with respect to the top plate moving direction.

[Fig. 5]

It is a block diagram which shows the structure of the central control part which concerns on 2nd Example of this invention.

[Fig. 6]

It is explanatory drawing which shows the situation which the projection direction is sequentially switched with respect to a subject.

[Fig. 7]

It is explanatory drawing which shows the situation which the sudden change of X-ray dose is prevented by setting the upper limit value for the X-ray tube output.

[Fig. 8]

It is explanatory drawing which shows the helical scan trajectory.

[Fig. 9]

It is a characteristic diagram which shows the change of the transmitted X-ray dose with respect to the angular position (projection angle) of an X-ray tube.

[Fig. 10]

It is explanatory drawing which shows the 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 Scanno image forming means 93 Scan range storage means 94 Scan condition determination means 95 Scan control means 97 Memories 98 Average transmitted dose calculation means