JPS6146141B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a radiation tomography apparatus that is capable of varying the measurement start point of radiation absorption data on a tomographic plane of a subject.
Computerized tomography (hereinafter referred to as computerized tomography) is one of the radiation tomography devices.
There is an X-ray tomography device called a CT scanner.

In this CT scanner, for example, an X-ray tube and an X-ray detection device are placed facing each other, and a pencil beam, for example, is emitted from the X-ray tube and moved in the same direction and at the same speed along the tomographic plane of the subject, and is moved once. This is repeated by changing the angle by one degree with respect to the tomographic plane each time, and thereafter, X-ray absorption data for various angles of the tomographic plane of the subject are collected while sequentially changing the angle. After collecting data in an angular range of 180°, this data is analyzed by a computer, the X-ray absorption rate at each position on the tomographic plane is calculated, and the tomographic plane is divided into gradations according to the absorption rate. The composition of each part of the tomographic plane can be analyzed in as many as 2,000 gradations in terms of X-ray absorption, so clear tomographic images can be obtained from soft tissues to hard tissues. can get.

Traverse-and-round type CT scanners, in which a radiation source such as an X-ray tube and a radiation detector work together in a linear motion relative to the subject during imaging, are generally referred to as first-generation or second-generation CT scanners. It is.

Figure 1 is a diagram showing the radiation path XR obtained by one linear movement of the first generation CT scanner. By linearly moving in one direction along the cross section, radiation paths XR are obtained at substantially equal intervals and in parallel along the tomographic plane of the subject A.

After performing such a linear movement once, the angle to the tomographic plane is changed and a similar linear movement is performed next. Figure 2 shows this situation.The angle is sequentially changed for each linear movement, and when the angle reaches 180 degrees, a radiation path can be obtained that projects the tomographic plane of the subject A from virtually all directions. Used for reproducing fault planes. There is also a type of traverse-and-round CT scanner called the second generation described above, which uses fan-shaped radiation having a relatively narrow spread angle. Examples of conventionally known specific technologies related to these include, for example, U.S. patent publications.
There are USP392431 and USP394634. The former is the first
The technology for the second generation CT scanner is shown. In all of these, the data collection range is an angular range smaller than 360°, and a tomographic image of the subject is obtained by applying radiation to the subject over this angular range.

The data collection range is theoretically data from all directions covering 360° on the tomographic plane to be imaged.
Even if data is not collected across 360 degrees, the data from the direction opposite to the already collected direction is the same, so data collection time can be shortened, data can be corrected, and calculations for reproducing tomographic planes (this is called image reconstruction calculations) ) etc., data collection is limited to the minimum necessary by collecting data only on one side in the direction where the same data is obtained, and data collection over the entire circumference is not normally performed. Therefore, the CT scanner must set the data collection area to the minimum necessary angle, which is less than 360°.

By the way, in all of these CT scanners, the absolute starting point of rotational movement relative to the subject is fixed at a specific position, and the subject is placed supine on the bed top. When the object is sent to a CT scanner, rotational movement is always started from the same angular position relative to the object, and data is collected over a predetermined angular range less than 360 degrees from this starting point.

This starting point is usually set at a position where it is easy to maintain balance, such as the apex (or the lowest point) of the rotation locus.

This is because the X-ray tube used as a radiation source is a heavy object, and in order to ensure the smooth operation of the mechanism that performs rotational movement and the stability of the load situation, a weight balance mechanism is installed to balance the weight that changes depending on the rotational position. This is due to the fact that it is not possible to ensure complete equilibrium, although the correction is made to maintain an equilibrium state.For example, the starting point is set at the apex above, where it is easiest to maintain balance, and the starting point is set after data collection is completed. This is because the X-ray tube is returned to its original position so that no unnecessary load is placed on the rotation mechanism or drive source of the X-ray tube when the X-ray tube is at rest.

In other words, since the X-ray tube is located at the starting point, it is easiest to start data collection from this position, and if data collection is always started from a fixed position, data correction etc. are easy and image replay is easy. Data collection always starts from the starting point, for reasons such as ease of configuration.

As a result, the following problems arise.

Figure 3 shows an example of the exposure dose distribution when the subject's head H is photographed over 180° using a CT scanner, where XT is the X-ray tube;
LS is the trajectory of the X-ray tube XT, B1 is the dose distribution area for 3 roentgens, B2 is the dose distribution area for 2 roentgens, B3 is the dose distribution area for 1 roentgen, and B4 is the dose distribution area for 1 roentgen.
The dose distribution area of 0.5 Roentgen is shown. As is clear from the figure, there is a difference in exposure dose of 3 roentgens and 0.5 roentgens, or 6 times, between areas near and far from the radiation source. This difference increases as the size of the region to be photographed becomes larger, such as the chest or abdomen.

On the other hand, the human body as a subject has parts that are vulnerable to radiation from a physiological standpoint, that is, parts where exposure to radiation should be suppressed as much as possible, such as the eyes, reproductive organs, and spine.

However, as mentioned above, in conventional CT scanners, the starting point of the rotational movement of the radiation source is fixed at a predetermined position, and when attempting to image a desired tomographic plane under a clinically required body position, There are many cases where the site that requires radiation exposure control must be located in an area with a strong radiation dose distribution.

Therefore, it has been impossible to perform imaging while avoiding areas where radiation exposure should be suppressed (away from the radiation source). Furthermore, in a CT scanner, a false image called a linear artifact occurs in the reconstructed image due to movement of the subject during imaging, making clinical diagnosis difficult.

The cause of this artifact is that regardless of the image reconstruction algorithm, e.g. convolution method, Fourier transform method, etc., the pattern of projection data indicating the intensity of radiation after passing through the object is shaped as it should be. However, this is due to changes due to the movement of the subject.

That is, when photographing a subject φ with a CT scanner, as shown in FIG. ₁ , P ₂ , P ₃ , P ₄ , P ₅ ...... are required.

However, if the object φ moves as shown by the dotted line during the process of obtaining the projection data P ₂ , the projection data will be given as P ₂ ′, and if φ did not move, it would originally be given. It shows a different pattern from the hot projection data _P2 .

As described above, this pattern change mainly generates linear artifacts in the _P2 projection direction with respect to the subject φ.

Furthermore, the strength of the artifacts that occur largely depends on the structure of the object that moves during imaging. In Fig. 4, it is assumed that unintended changes in the shape of the projection data are the cause of artifacts, but the size of this projection data is determined by the radiation absorption rate of the components included in the subject. from,
If an object whose radiation absorption coefficient is significantly larger than the average radiation absorption coefficient of the subject, such as bones, or extremely small, such as air vesicles, moves during imaging, it will have a large effect on the projection data. Therefore, it is understood that the strength of the artifact depends on the structure of the subject.

The above corresponds to the fact that even when the same subject is imaged, there are differences in artifacts caused by the movement of the subject, etc., depending on the rotation range of the radiation source relative to the subject.

For example, as shown in Fig. 5, when imaging the lateral plane (frontal plane) of the head and intending to diagnose the brain parenchyma D, the direction of the arrow E is the temporally different projection data of the imaging start point and end point. In addition, since there are small bones with complex shapes such as teeth in the mandibular region F, artifacts G caused by these bones with a significantly high radiation absorption coefficient occur, and the brain parenchyma D is affected. Often hinders diagnosis.

In such a case, by shifting the radiation exposure start position range of the radiation source (X-ray tube XT) by about 90 degrees, it is possible to remove the artifact in the brain parenchymal region D.

However, in conventional CT scanners, the starting point of the rotational movement of the radiation source with respect to the subject is fixed, and the data collection start position, that is, the radiation exposure starting position, is also set at this starting point, so the body position required clinically can be adjusted. In order to reduce artifacts in areas that are necessary for clinical diagnosis, and to avoid areas where exposure should be avoided from strong radiation distribution areas as much as possible, it is possible to move the data collection start point. I couldn't do it.

The present invention was made in view of the above circumstances, and
By making it possible to shift the starting point of the rotational movement of the CT scanner arbitrarily, the occurrence of artifacts can be suppressed even when the starting point of the rotational movement of the radiation source is fixed at a fixed position as in the past. Another object of the present invention is to provide a radiation tomography apparatus that can suppress radiation exposure to areas that should be avoided as much as possible.

FIG. 6 is a diagram showing a schematic configuration of this device, in which 1 is a ring-shaped base rotatably supported by a support device (not shown), 2 is an X-ray tube, and 3 is an X-ray detector. The X-ray tube 2 and the X-ray detector 3 are mounted on the base 1 facing each other. 4
is an electric motor, and a cable 5 such as a chain or a synchronized belt is stretched between the electric motor 4 and the base 1, and the rotation of the electric motor 4 causes the base 1 to
is now being rotated. 6 is the base 1
A setting device 7 is used to set the rotation angle position for starting the exposure of the upper X-ray tube 2, and a setting device 7 controls the drive of the electric motor 4, and the direction of the X-ray tube 2 is set by the setting device 6.
This is a control device that starts X-ray exposure when the set angle is reached.

This apparatus with the above configuration places the subject A in the central hollow part of the ring-shaped base 1 and performs imaging.
The radiation direction of the wire tube 2 is set, for example, with the horizontal right direction as a reference angle, and a desired angle with respect to this reference angle is set using the setter 6. Then, when a starting command (not shown) is given to start the control device 7, the electric motor 4 is rotated by the output from the control device 7, and the rotational force is transmitted to the base 1 via the cable 5. As a result, the base 1 starts rotating. As the base body 1 rotates, the X-ray tube 2 is also moved together. The rotation angle of the base 1 is detected by a detector (not shown), and the detected rotation angle is compared with a setting value of the setting device 6 by the control device 7. When the detected rotation angle reaches the set value, the control device 7 gives an X-ray exposure start command to the X-ray tube 2, and thereafter, for example, from the exposure start point.
X-ray irradiation is performed until it rotates 180 degrees.

Thus, data can be collected over a range of 180° from the desired location. Therefore, even if the starting point of the rotational movement of the X-ray tube 2 is fixed at a fixed position as in the past, imaging can be started from any position depending on the situation, so the occurrence of artifacts can be suppressed. Since high-quality photographs can be obtained, it is possible to not only make accurate diagnoses, but also reduce the amount of radiation in areas where radiation exposure should be avoided as much as possible.

In this way, we have made it possible to arbitrarily change the data collection start point of the CT scanner, making it possible to prevent artifacts from occurring and make accurate diagnoses.Also, it is necessary to suppress radiation exposure as much as possible. It is possible to provide a radiation tomography apparatus having excellent features such as being able to reduce radiation exposure to areas that need to be exposed to radiation.

It should be noted that the present invention is not limited to the embodiments described above and shown in the drawings, but can be implemented with appropriate modifications within the scope of the gist, and in addition to the automatic control system as in the above embodiments, A manual method may be used in which the position is moved and set by manually operating gears, etc.

[Brief explanation of the drawing]

Fig. 1 shows the radiation path obtained by one linear movement of the CT scanner, Fig. 2 shows the change in the radiation path caused by rotational movement, and Fig. 3 shows the radiation path obtained by a single linear movement of the CT scanner. A diagram showing an example of the radiation dose distribution when photographing over 180°, Figure 4 is a diagram showing the projection data distribution for each direction when the subject is projected from each direction, and Figure 5 is a diagram showing the occurrence of artifacts. FIG. 6, which is a diagram for explanation, is a schematic configuration diagram showing an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Base body, 2... X-ray tube, 3... X-ray detector, 4... Electric motor, 6... Setting device, 7... Control device.

Claims (1)

[Claims] 1 A radiation source and a radiation detector are placed facing each other, and the radiation source is rotationally driven while emitting radiation along the cross section of the object to be inspected, so that each In a radiation tomography apparatus that collects and analyzes radiation absorption data from different directions to reconstruct the tomographic image, a setting is made to set a desired angle with respect to the reference angle using a predetermined rotational position of the radiation source as a reference angle. A radiation tomography apparatus comprising: a control means for detecting a rotation angle of the radiation source and starting radiation exposure from the radiation source when the desired rotation position is reached.