JP2000023970A - X-ray ct device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain images, through a small amount of computations, the calculation of the displacement position of a shielding plate within a collimator and the detection of a channel which is shielded so that projection data are abruptly changed, in a CT device irradiating a set area with X-rays to obtain images. SOLUTION: For a shielding plate displacement position in a collimator for irradiating a set area with X-rays, geometrical computations for all projection angles are not performed but an approximate curve of displacement positions at all of the projection angles is calculated from the results of calculations at several parts and the displacement positions are calculated therefrom to reduce the amount of computations. The channel of abruptly changing projection data is detected using threshold processing or filter processing without increasing the amount of computations.

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 capable of reducing exposure to radiation by attenuating X-rays outside a set area by collimating the X-rays in a channel direction.
Related to the device.

[0002]

2. Description of the Related Art X-ray CT apparatuses have already been widely used in medical treatment and the like, and various usages have been made by users. For example, recently, when performing tissue examination or treatment of a lesion percutaneously, an X-ray CT apparatus is used as a puncture guide. As described above, by performing the tissue examination and treatment of the lesion under the guide of the X-ray CT apparatus, the operation time is reduced, and it is regarded as effective because the accuracy is improved.

[0003] By the way, as a guide method using such an X-ray CT apparatus, puncturing and CT imaging are alternately and intermittently repeated so as to confirm the position of the tip of a puncture needle, or CT imaging. Is performed continuously,
There is a method of sequentially displaying images so that the position of the puncture needle can be immediately confirmed. Among them, especially in the latter method,
Since a tomographic image can be obtained in real time, there is an advantage that the operation time is further reduced.

[0004]

However, in the above-described method in which CT imaging is performed intermittently or continuously, a patient or an operator needs to reduce the amount of X-ray exposure due to the intermittent or continuous CT imaging. Growth is a problem.

[0005] In order to solve this problem, the present applicant has filed a patent application (Japanese Patent Application No. 9-199185). Japanese Patent Application No. 9
No. 199185 proposes a method of irradiating only a set region with X-rays using a collimator whose opening angle can be controlled in the channel direction. However, irradiating the X-ray only to the set area during rotation of the X-ray CT apparatus, in other words, finding the position of the shielding plate that attenuates the X-ray requires a very complicated calculation. Also, when calculating the boundary channels inside and outside the irradiation area of the projection data attenuated by the shielding plate, the calculation is complicated and the calculation amount is significantly increased.

SUMMARY OF THE INVENTION In view of the above-mentioned problems in the prior art, an object of the present invention is to accurately calculate a position of a shielding plate and a calculation of a boundary channel of an irradiation area without increasing the calculation amount.

[0007]

According to the present invention, a region of interest is set in advance, and when the region of interest is scanned, an X-ray is irradiated so that only the region of interest can be irradiated during the scan. In an X-ray CT apparatus for performing CT measurement of a region of interest by performing attenuation control of X-rays outside the region of interest in the channel direction, the X-ray CT apparatus includes a plurality of projection angles discretely selected from all projection angles. Only the attenuation position in the channel direction to be illuminated is given, and using these attenuation positions,
An attenuation position in the channel direction to be irradiated only to the region of interest at all projection angles during scanning is obtained, and attenuation control of X-rays is sequentially performed in real time so as to be a corresponding attenuation position over the entire projection angle. The disclosed X-ray CT apparatus is disclosed.

Further, according to the present invention, a boundary channel corresponding to the attenuation position is given, and data not measured at the time of image reconstruction is obtained for correction or the like, thereby reducing artifacts. Further, according to the present invention, the boundary channel is obtained from a sudden change position on the measurement data.

According to the present invention, in order to achieve the above object, the attenuation position, that is, the movement position of the shielding plate is not calculated geometrically at all projection angles, but the movement distance of a certain number of projection angles is obtained. For other projection angles, a filtering process is performed to obtain an approximate value from the moving distance obtained in the above-mentioned several places, and to obtain a boundary position inside and outside the region of interest of the projection data to be detected by shielding the X-ray. Or by threshold processing without using complicated calculations.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. The scanning method proposed in Japanese Patent Application No. 9-199185 or the like is a scanning method in which the subject is not irradiated with X-rays outside the selected and set range in order to suppress the exposure dose to the patient and the operator. FIG. 1 shows an example in which a CT image 12 of a subject 2 is displayed on a CRT display 10 and a partial area (for example, a circular area) 11 of the liver is selected and set. Thus, when irradiating only part of the liver, which is an example of a circular region, with X-rays,
An irradiation area such as a circle is set based on the image 12 once obtained by a normal scan (pre-scan). The left and right channel collimators 4 and 5 operate based on this area (see FIG. 2, the structure of the channel collimator is described in Japanese Patent Application No. 9-199185, etc.). Do not irradiate (attenuate) X-rays from 1.

As shown in FIG. 3 (a), the X-ray CT apparatus fetches data via the X-ray detector 2 while rotating around the subject 3 by 360 ° as shown in the direction of the arrow. In order to irradiate the region 11 with X-rays, the channel collimators 4 and 5 move independently during scanning. FIG. 3B shows the movement position (attenuation position) of the shield plate in the channel collimators 4 and 5, the horizontal axis shows the projection angle, and the vertical axis shows the movement distance (of the X-ray fan beam). The center is 0, the right side is a plus direction, and the left side is a minus direction.)

Japanese Patent Application No. Hei 9-199185 describes that a geometric operation is performed at all projection angles in order to obtain the shield plate moving distance shown in FIG. 3B, but this operation requires an inverse trigonometric function. If this calculation is performed for all projection angles, the calculation amount will be greatly increased. For details of the calculation formula, refer to Japanese Patent Application No. Hei 9-199185.

In the present invention, the curve shown in FIG. 3B is approximated from several projection data, and the method is as follows. As shown in FIG. 4, first, the collimator movement positions at several places are calculated as initial conditions. In the drawing of FIG. 4, four points of the X-ray sources a, b, c, and d having different phases by 90 degrees are taken as examples. FIG. 5 is a plot of data obtained from these four points on a graph in which the horizontal axis represents the projection angle and the vertical axis represents the movement distance of the shielding plate. These four points can be obtained by a geometric operation described in Japanese Patent Application No. 9-199185.

When the curve (right shield plate) of FIG. 5 is approximated by a sine function, the horizontal axis represents the projection angle x, and the vertical axis represents the moving distance y.
The general formula can be represented by the following equation (1).

(Equation 1) Here, A is the amplitude of the sine function, and φ is the phase angle. Two of the four initial conditions, i.e. the X-ray source a, conditions in the position of _{b (x a, y a)} , (x b, y b) number based on 1 unknowns A, calculating the φ Can be obtained from the simultaneous equations of the following equation (2).

(Equation 2) When A and φ are obtained by transforming Equation 2, the following Equation 3 is obtained.

(Equation 3) By substituting the result into Equation 1, the general equation of Equation 1 can be specified.

Although it is possible to approximate the obtained equation for all projection angles, the accuracy of the approximate curve is not very high. In addition to the conditions at the positions of the X-ray sources a and b, there are conditions (x _c , y _c ) and (x _d , y _d ) of c and d. These are defined as sections ab, bc, cd, d-
If the approximated curves are obtained by dividing into four sections a, a more accurate approximated curve can be obtained. FIG. 6 illustrates this. As described above, the movement positions at all the projection angles can be obtained by the approximate curves without increasing the amount of calculation based on the movement positions at several projection angles.

In the above description, an approximate curve is obtained as an example using a sine function of a trigonometric function. However, the approximation may be another trigonometric function, or may be performed using a quadratic function or a higher-order function. can do.

When X-rays are irradiated only to the set area, there is no data outside the set channel (indicated by the broken line), and the projection data has a sudden change at the boundary of the set channel as shown in FIG. I will be. The reason that the rapidly changing boundary channel of the data indicating the inside and outside of the region of interest is different for each projection data is that as shown in FIG. Can not be irradiated. In other words, the movement of the shielding plate at each projection angle means that the boundary channel, which changes rapidly, also differs at each projection angle. Here, at the time of image reconstruction, data outside the boundary channel is obtained in some form (correction or the like), and this is also used for reconstruction, whereby artifacts can be reduced. In this case, a boundary channel is specified, data outside the channel is obtained by correction or the like, and then an image is reconstructed. As a correction method for reconstructing an image in such a case, a method described in Japanese Patent Application Nos. 9-199185 and 9-183845 may be used.

The boundary channel is disclosed in Japanese Patent Application No. 9-199185.
The boundary position can be obtained from the collimator movement position calculated by the collimator movement position calculation described in (1). However, a large amount of calculation is required as described above. In the present embodiment, a boundary channel in which data changes abruptly by a geometric calculation can be obtained without increasing the amount of calculation by using the movement position calculation result based on the above-described approximate curve.

The method for determining the boundary channel can be further simplified. This will be described below. This embodiment does not determine the boundary position from the moving position of the collimator,
This is a method of detecting a suddenly changing channel from the characteristics of the captured projection data. One method is to set a threshold. Since this is blocked as shown in FIG. 8 (if not blocked, the shape of the projection data is indicated by a broken line), the difference value of the data of the adjacent channel is significantly different from the difference value of the other channels. . Here in the channel at the boundary r _a, the channel of the next r _b
If you and to further the channel next to a r _c, r _a and r
difference value _b than the difference value of r _b and r _c, it is apparent from becoming a large value significantly absolute value. In the present embodiment, a threshold value is set for this difference value, and a channel that changes rapidly is detected. Now, if the threshold is determined to be RR, the equation becomes a simple equation as shown in Equation 4.

(Equation 4) If the difference value between adjacent channels is equal to or more than RR (threshold), Equation 4 is satisfied, and the channel is regarded as a boundary channel, and a boundary can be detected. As a method of detecting a boundary channel in addition to the threshold value processing, there is a method of using filter filtering. Although not described using an equation, a channel whose value changes drastically can be detected using a Laplacina filter or the like.

As described above, a boundary channel is detected, data outside the region of interest is corrected, and an image is reconstructed from these data, whereby an image with less artifacts can be obtained.

[0021]

According to the present invention, in an X-ray CT apparatus in which X-rays are not irradiated as much as possible outside a set area, a highly accurate calculation of a movement position of a shielding plate in a collimator for limiting an irradiation area is performed. By using a simple approximation, the amount of calculation can be reduced, and a practical movement position can be calculated.
Also, the detection of a channel whose data changes suddenly due to occlusion can be realized by a method with a relatively small amount of calculation.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an X-ray irradiation area set on a tomographic image.

FIG. 2 is an explanatory diagram of shielding outside an irradiation area.

FIG. 3 is an explanatory view of a movement of a shielding plate.

FIG. 4 is an explanatory diagram of initial conditions of a shielding plate.

FIG. 5 is an explanatory diagram of a shielding plate moving position approximation curve.

FIG. 6 is an explanatory diagram of a shielding plate moving position approximation curve.

FIG. 7 is an explanatory diagram of the shape of projection data when the image is not occluded and when the image is occluded.

FIG. 8 is an explanatory diagram of a difference value between adjacent channels.

[Explanation of symbols]

 Reference Signs List 1 CRT display 2 Subject 3 Region of interest 4,5 Collimator

Claims (1)

[Claims] 1. A region of interest is set in advance, and during scanning of the region of interest, X-rays outside the region of interest in the X-ray channel direction are provided so that only the region of interest can be irradiated with X-rays during the scan. By performing line attenuation control, the CT of the region of interest is
In an X-ray CT apparatus for measurement, attenuation positions in a channel direction to be irradiated only on the region of interest at a plurality of projection angles discretely selected from all projection angles are given, and these attenuation positions are determined. The attenuation positions in the channel direction to be irradiated only to the region of interest at all the projection angles during the scan are obtained, and the attenuation control of the X-ray is performed in real time so as to be the corresponding attenuation position over the entire projection angle. An X-ray CT system that is to be performed one after another.