JP4509493B2 - X-ray CT image capturing method and X-ray CT apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray CT (Computed Tomography) imaging method and an X-ray CT apparatus, and more specifically, an X-ray exposure dose at a highly radioactive site near the body surface of a subject. The present invention relates to a line CT image imaging method and an X-ray CT apparatus.
[0002]
[Prior art]
Conventionally, an X-ray CT apparatus including a channel collimator or an attenuation body that suppresses X-ray irradiation to a region of interest is known (for example, see Patent Document 1).
[0003]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-28201 [0004]
[Problems to be solved by the invention]
According to the conventional X-ray CT apparatus, for example, the X-ray exposure dose of the eyeball can be reduced using the channel collimator or the attenuation body.
However, there is a problem that the configuration is complicated because a channel collimator or an attenuation body and a control means for taking it out and in are required.
Therefore, an object of the present invention is to provide an X-ray CT image capturing method and an X-ray CT apparatus capable of reducing the X-ray exposure dose of a highly radioactive site near the body surface of a subject with a simple configuration. There is to do.
[0005]
[Means for Solving the Problems]
In a first aspect, the present invention divides the gantry angle into at least two types of angle ranges according to the distance between the X-ray focal point and a highly radioactive site near the body surface of the subject, X-ray irradiation output at a gantry angle included in an angle range of near type is made smaller than an X-ray irradiation output at a gantry angle included in an angle range of far distance, and projection data is collected. An X-ray CT image capturing method is provided.
In the X-ray CT image capturing method according to the first aspect, the X-ray irradiation output is reduced at the gantry angle where the distance between the X-ray focal point and the highly radioactive part near the body surface of the subject is close. It is possible to reduce the amount of X-ray exposure at a highly radioactive site near the body surface of the subject. In order to reduce the X-ray irradiation output, the tube current only needs to be lowered. Therefore, the channel collimator or the attenuation body and the control means for taking it in and out are not required, and the configuration is simple.
[0006]
According to a second aspect, the present invention provides an X-ray CT image capturing method having the above-described configuration, wherein the first type of angular range that is close to the distance and a second type of angular range that faces the first type of angular range. An angle range is divided into three types of angle ranges: a first type of angular range and a third type of angular range between the second type of angular range and the first type of angular range. The X-ray irradiation output at the gantry angle included in the angle range is set as the first X-ray irradiation output, and the X-ray irradiation output at the gantry angle included in the second type angle range is set as the second X-ray irradiation output. When the X-ray irradiation output at the gantry angle included in the third type angle range is the third X-ray irradiation output, the first X-ray irradiation output <the third X-ray irradiation output <the first An X-ray CT image imaging method is provided, characterized in that the X-ray irradiation output of 2 is used.
In the X-ray CT image capturing method according to the second aspect, the X-ray irradiation output is reduced at the gantry angle where the distance between the X-ray focal point and the highly radioactive part near the body surface of the subject is close. It is possible to reduce the amount of X-ray exposure at a highly radioactive site near the body surface of the subject. Further, since the X-ray irradiation output is increased at the gantry angle facing the gantry angle, the influence on the image quality due to the reduction in the X-ray irradiation output can be compensated for, and the deterioration of the image quality can be suppressed.
In the case of a fan beam, a gantry angle that is 180 degrees different from the center line of the fan beam may be used as a gantry angle that faces a certain gantry angle.
[0007]
In a third aspect, the present invention provides the X-ray CT image capturing method configured as described above, wherein the first X-ray irradiation output is the smallest at the center of the angle range of the first type and the third X-ray output at both ends. The second X-ray irradiation output is the largest at the center of the second type of angular range and the third X-ray irradiation output at both ends. There is provided an X-ray CT image capturing method characterized in that the third X-ray irradiation output varies substantially without steps so as to be connected, and the third X-ray irradiation output is constant in the third type angle range.
In the X-ray CT image capturing method according to the third aspect, since the X-ray irradiation output changes substantially without a step, it is possible to suppress deterioration in image quality due to the change of the X-ray irradiation output.
[0008]
In a fourth aspect, the present invention provides an X-ray CT image radiographing method having the above-described configuration, wherein the projection data is weighted according to the X-ray irradiation output when the projection data is obtained, and image reconstruction is performed. An X-ray CT image capturing method is provided.
In the X-ray CT image capturing method according to the fourth aspect, a small weight is given to projection data obtained with a small X-ray irradiation output, and a large weight is given to projection data obtained with a large X-ray irradiation output. As a result, signal-to-noise characteristics can be improved and deterioration in image quality can be suppressed.
[0009]
In a fifth aspect, the present invention provides an X-ray CT image capturing method characterized in that, in the X-ray CT image capturing method configured as described above, the highly radioactive portion is an eyeball or a breast.
In the X-ray CT image capturing method according to the fifth aspect, the X-ray exposure dose of the eyeball or breast can be reduced.
[0010]
In a sixth aspect, the present invention provides an X-ray source, a detector, scanning means for collecting projection data at multiple gantry angles while rotating the X-ray source around the subject, and at least two gantry angles. An X-ray irradiation output at a gantry angle included in the first type of angle range when being divided into types of angle ranges is made smaller than an X-ray irradiation output at a gantry angle included in the second type of angle range. There is provided an X-ray CT apparatus comprising: a beam irradiation output control unit; and an image reconstruction unit that generates a CT image based on collected projection data.
In the X-ray CT apparatus according to the sixth aspect, the X-ray CT image capturing method according to the first aspect can be suitably implemented.
[0011]
In a seventh aspect, the present invention provides the X-ray CT apparatus configured as described above, wherein the X-ray irradiation output control means is a second type opposite to the first type angular range and the first type angular range. The first type of angle range is divided into three types of angle ranges: a first type of angular range and a third type of angular range between the second type of angular range and the first type of angular range. The X-ray irradiation output at the gantry angle included in the angle range is set as the first X-ray irradiation output, and the X-ray irradiation output at the gantry angle included in the second type angle range is set as the second X-ray irradiation output. When the X-ray irradiation output at the gantry angle included in the third type angle range is the third X-ray irradiation output, the first X-ray irradiation output <the third X-ray irradiation output <the first An X-ray CT apparatus characterized by having an X-ray irradiation output of 2 is provided.
In the X-ray CT apparatus according to the seventh aspect, the X-ray CT image capturing method according to the second aspect can be suitably implemented.
[0012]
In an eighth aspect, the present invention provides the X-ray CT apparatus configured as described above, wherein the first X-ray irradiation output is the smallest at the center of the angle range of the first type and the third X-rays at both ends. The second X-ray irradiation output is changed so as to be substantially connected to the irradiation output so that the second X-ray irradiation output is the largest at the center of the second type angle range and is connected to the third X-ray irradiation output at both ends. The X-ray CT apparatus is characterized in that the third X-ray irradiation output changes substantially without any step, and the third X-ray irradiation output is constant in the third type angle range.
In the X-ray CT apparatus according to the eighth aspect, the X-ray CT image capturing method according to the third aspect can be suitably implemented.
[0013]
In a ninth aspect, the present invention provides the X-ray CT apparatus having the above-described configuration, wherein the image reconstruction is performed by weighting the collected projection data according to an X-ray irradiation output when the projection data is obtained. There is provided an X-ray CT apparatus comprising weighting means for passing to the means.
In the X-ray CT apparatus according to the ninth aspect, the X-ray CT image capturing method according to the fourth aspect can be suitably implemented.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to embodiments shown in the drawings. Note that the present invention is not limited thereby.
[0015]
-First embodiment-
FIG. 1 is a configuration block diagram showing an X-ray CT apparatus 100 according to the first embodiment.
The X-ray CT apparatus 100 includes an operation console 1, a table apparatus 10, and a scanning gantry 20.
[0016]
The operation console 1 includes an input device 2 that receives input from an operator, a central processing unit 3 that executes image generation processing, a projection data collection buffer 5 that collects projection data acquired by the scanning gantry 20, and a projection data. A CRT 6 for displaying the reconstructed CT image and a storage device 7 for storing a program, projection data, and CT image are provided.
[0017]
The table device 10 includes a cradle 12 that puts a subject and puts it in and out of a bore (cavity) of the scanning gantry 20. The cradle 12 is driven by a motor built in the table apparatus 10.
[0018]
The scanning gantry 20 rotates an X-ray tube 21, an X-ray controller 22, a collimator 23, a detector 24, a DAS (Data Acquisition System) 25, and the X-ray tube 21 around the body axis of the subject. And a control interface 29 for exchanging control signals and the like with the operation console 1 and the table device 10.
[0019]
FIG. 2 is an explanatory diagram showing the relationship between the gantry angle θ and the projection angle φ of the X-ray beam.
The gantry angle θ is a rotation angle of the X-ray tube 21 rotating around the rotation center O. Here, the gantry angle when the center line of the fan beam F is vertically downward is θ = 0.
The projection angle φ of the X-ray beam is an angle of the X-ray beam incident on each channel of the detector 24. Here, the projection angle when the X-ray beam is vertically downward is φ = 0.
When the fan angle is 2γm, the projection angle of the X-ray beam incident on the center channel (channel number i = 0) of the detector 24 is φ = θ, and both end channels (channel number i = m, The projection angle of the X-ray beam incident on −m) is φ = θ + γm and φ = θ−γm.
[0020]
FIG. 3 is a flowchart showing the operation of the X-ray CT image photographing process by the X-ray CT apparatus 100.
In step S <b> 1, the operator operates the input device 2 to select “exposure reduction mode” or “normal mode”.
In step S2, when the selected mode is “exposure reduction mode”, the process proceeds to step S3, and when the selected mode is “normal mode”, the process proceeds to step S6.
[0021]
In step S3, projection data is collected by performing a scan by exposure-reduced X-ray irradiation. This exposure-reducing X-ray irradiation will be described next with reference to FIGS.
As shown in FIGS. 4 and 5, a protected site P including a left eyeball EL and a right eyeball ER of a subject H, which is a highly radioactive site near the body surface, is assumed. Further, it is assumed that the subject H is scanned in the supine position. In addition, the distances between the X-ray focal point and the left eyeball EL and the right eyeball ER are simply represented by the distances L (θ1) and L (θ2) between the X-ray tube 21 and the protection site center Pc.
As can be seen by comparing FIG. 4 and FIG. 5, the distance L (θ1) at the gantry angle θ1 in the vicinity of θ = 0 is smaller than the distance L (θ2) at the gantry angle θ2 in the vicinity of θ = π. Further, at the gantry angle θ1 in the vicinity of θ = 0, the X-ray beam hits the protection site P with almost no attenuation, but at the gantry angle θ2 in the vicinity of θ = π, the X-ray beam hits the protection site P after being considerably attenuated. .
Therefore, as shown in FIG. 6, an angle range −βu ≦ θ ≦ βu in the vicinity of θ = 0 is set as a first type of angle range R1, and an angle range π−βu ≦ θ ≦ π + βu in the vicinity of θ = π is set. The second type of angular range R2 is assumed, and the angular range between the first type of angular range R1 and the second type of angular range R2 is referred to as a third type of angular range R3.
[0022]
Then, as shown in FIG. 7, the first X-ray irradiation output at the gantry angles 0 ≦ θ ≦ βu and π−βu ≦ θ <2π included in the first type angle range R1 is expressed as the first type. Gantry angles βu <θ <π−βu and π + βu <θ, which are the smallest in the center of the angle range R1, that is, θ = 0, which are the smallest and both ends, ie, θ = βu and θ = π−βu, are included in the third type angle range R3 <It changes without a level | step difference substantially so that it may lead to the 3rd X-ray irradiation output in (pi- (beta) u (it is changing linearly here, but you may change nonlinearly). Further, the second X-ray irradiation output at the gantry angle π−βu ≦ θ ≦ π + βu included in the second type angle range R2 is the largest at the center of the second type angle range R2, that is, θ = π. And the third X-ray irradiation output at both ends, that is, θ = π−βu and θ = π + βu and gantry angles βu <θ <π−βu and π + βu <θ <π−βu included in the third type angle range R3. So as to compensate for the fact that the X-ray irradiation output is reduced by the first X-ray irradiation output. Further, the third X-ray irradiation output at the gantry angles βu <θ <π−βu and π + βu <θ <2π−βu included in the third type angle range R3 is constant.
[0023]
Returning to FIG. 3, in step S4, preprocessing (offset correction, logarithmic correction, X-ray dose correction, sensitivity correction) is performed on the projection data.
[0024]
In step S5, as shown in FIG. 8, a weighting process for applying a weight according to the X-ray irradiation output when the projection data is obtained is performed on the projection data. Then, the process proceeds to step S8.
[0025]
In step S6, normal X-ray irradiation scans are performed to collect projection data. That is, as shown in FIG. 9, the projection data is collected with the X-ray irradiation output constant regardless of the gantry angle θ.
In step S7, preprocessing (offset correction, logarithmic correction, X-ray dose correction, sensitivity correction) is performed on the projection data. Then, the process proceeds to step S8.
[0026]
In step S8, an image reconstruction process is performed on the projection data to generate a CT image.
In step S9, the CT image is post-processed so as to be suitable for display.
In step S10, the CT image is displayed on the CRT 6.
[0027]
According to the X-ray CT apparatus 100 of the first embodiment, since the X-ray irradiation output is reduced at the gantry angle where the distance between the eyeballs EL and ER and the X-ray focal point is close, the X-ray exposure dose of the eyeballs EL and ER. Can be reduced. In order to reduce the X-ray irradiation output, the tube current only needs to be lowered. Therefore, the channel collimator or the attenuation body and the control means for taking it in and out are not required, and the configuration is simple.
In addition, a small weight is given to projection data obtained with a small X-ray irradiation output, and a large weight is given to projection data obtained with a large X-ray irradiation output, thereby improving the signal-to-noise characteristics. A reduction in image quality can be suppressed.
[0028]
-Second Embodiment-
In the second embodiment, as shown in FIG. 10, the angle range −βu ≦ θ ≦ βu in the vicinity of θ = 0 is set as the first type of angle range R1, and the angle range π−βu in the vicinity of θ = π. −2γm ≦ θ ≦ π + βu + 2γm is a second type of angular range R2, and an angular range between the first type of angular range R1 and the second type of angular range R2 is a third type of angular range R3. To do.
[0029]
Then, as shown in FIG. 11, the first X-ray irradiation output at the gantry angles 0 ≦ θ ≦ βu and π−βu ≦ θ <2π included in the first type angle range R1 is “small”, The second X-ray irradiation output at the gantry angle π−βu−2γm ≦ θ ≦ π + βu + 2γm included in the second type angle range R2 is set to “large”, and the gantry angle included in the third type angle range R3. The third X-ray irradiation output at βu <θ <π−βu−2γm and π + βu + 2γm <θ <2π−βu is set to “medium”.
[0030]
In addition, as shown in FIG. 12, the image reconstruction process is performed by assigning a weight according to the X-ray irradiation output when the projection data is obtained to the projection data.
[0031]
According to the X-ray CT apparatus of the second embodiment, for each channel of the detector 24, projection of an opposite view with an X-ray irradiation output of “large” with respect to projection data with an X-ray irradiation output of “small”. Since data can be obtained, the influence on the image quality caused by reducing the X-ray irradiation output can be more highly compensated.
[0032]
-Other embodiments-
In the same manner as described above, for example, the amount of X-ray exposure to the breast can be reduced.
[0033]
【The invention's effect】
According to the X-ray CT image capturing method and the X-ray CT apparatus of the present invention, it is possible to reduce the amount of X-ray exposure to a highly radiation-sensitive part near the body surface of the subject, such as an eyeball or a breast.
[Brief description of the drawings]
FIG. 1 is a configuration block diagram showing an X-ray CT apparatus according to a first embodiment.
FIG. 2 is an explanatory diagram showing a relationship between a gantry angle θ and an X-ray beam projection angle φ.
FIG. 3 is a flowchart showing an operation of an X-ray CT image photographing process by the X-ray CT apparatus according to the first embodiment.
FIG. 4 is an explanatory diagram showing a distance between an X-ray tube and an eyeball at a gantry angle in the vicinity of θ = 0.
FIG. 5 is an explanatory diagram showing a distance between an X-ray tube and an eyeball at a gantry angle in the vicinity of θ = π.
FIG. 6 is an explanatory diagram showing divisions of an angle range of gantry angles according to the first embodiment.
FIG. 7 is a graph showing the relationship between the gantry angle and the X-ray irradiation output in the exposure-reducing X-ray irradiation mode according to the first embodiment.
FIG. 8 is a graph showing a relationship between a gantry angle and a weight in an exposure reduction X-ray irradiation mode according to the first embodiment.
FIG. 9 is a graph showing a relationship between a gantry angle and an X-ray irradiation output in a normal mode.
FIG. 10 is an explanatory diagram showing divisions of an angle range of gantry angles according to the second embodiment.
FIG. 11 is a graph showing the relationship between the gantry angle and the X-ray irradiation output in the exposure-reducing X-ray irradiation mode according to the second embodiment.
FIG. 12 is a graph showing a relationship between a gantry angle and a weight in an exposure reduction X-ray irradiation mode according to the second embodiment.
[Explanation of symbols]
20 Scanning gantry 21 X-ray tube 22 X-ray controller 24 Detector 26 Rotation controller 100 X-ray CT apparatus

Claims (4)

An x-ray source, a detector, and scanning means for collecting projection data at multiple gantry angles while rotating the x-ray source around the subject;
When the gantry angle for one rotation is divided into at least two types of angle ranges of a first type of angle range and a second angle range including an angle range opposite to the first angle range, X-ray irradiation output control means for making the X-ray irradiation output at the gantry angle included in the first type angle range smaller than the X-ray irradiation output at the gantry angle included in the second type angle range;
Against acquired projection data, when X-ray irradiation output when obtain the projection data is larger than become smaller weight when less than such a greater weight, facing corresponding to the X-ray radiation output A weighting means for weighting at a gantry angle ;
Image reconstruction means for generating a CT image using projection data for one rotation weighted by the weighting means ,
The X-ray irradiation output control means has an X-ray irradiation whose X-ray irradiation output at a gantry angle included in the first type angle range is smaller than an X-ray irradiation output that is not substantially weighted by the weighting means. The X-ray irradiation output at the gantry angle included in the second type angle range is larger than the X-ray irradiation output that is not substantially weighted by the weighting means. X-ray CT apparatus characterized by some. An x-ray source, a detector, and scanning means for collecting projection data at multiple gantry angles while rotating the x-ray source around the subject;
Input means capable of selecting either normal mode or exposure reduction mode;
When the normal mode is selected, the gantry angle for one rotation is set as a constant X-ray irradiation output,
When the exposure reduction mode is selected, the gantry angle for one rotation is set to at least two of a first type angle range and a second angle range including an angle range opposite to the first angle range. X-ray irradiation output at a gantry angle included in the first type of angle range is divided into X-ray irradiation output at a gantry angle included in the second type of angle range when the angle range is classified into types. X-ray irradiation output control means for reducing,
When the exposure reduction mode is selected, the acquired projection data has a smaller weight when the X-ray irradiation output when the projection data is obtained is smaller, and a larger weight when larger. , Weighting means for performing weighting at an opposing gantry angle corresponding to the X-ray irradiation output,
When the normal mode is selected, the projection data collected with the constant X-ray irradiation output is used. When the exposure reduction mode is selected, one rotation weighted by the weighting means is applied. Image reconstruction means for generating a CT image using the projection data of the minute ,
The X-ray irradiation output control means has an X-ray irradiation whose X-ray irradiation output at a gantry angle included in the first type angle range is smaller than an X-ray irradiation output that is not substantially weighted by the weighting means. The X-ray irradiation output at the gantry angle included in the second type angle range is larger than the X-ray irradiation output that is not substantially weighted by the weighting means. X-ray CT apparatus characterized by some. The X-ray CT apparatus according to claim 1 or 2 ,
The X-ray irradiation output control means includes a first type of angular range, a second type of angular range opposite to the first type of angular range, the first type of angular range, and the second type. Are divided into three types of angle ranges, the third type of angle range between the first and second types of angle ranges, and the X-ray irradiation output at the gantry angle included in the first type of angle range is the first X-ray. The X-ray irradiation output at the gantry angle included in the second type angle range is set as the second X-ray irradiation output, and the X-ray at the gantry angle included in the third type angle range is set as the irradiation output. An X-ray CT apparatus characterized in that when the irradiation output is a third X-ray irradiation output, the first X-ray irradiation output <the third X-ray irradiation output <the second X-ray irradiation output. The X-ray CT apparatus according to claim 3 ,
The first X-ray irradiation output is the smallest at the center of the angle range of the first type and changes substantially without steps so as to be connected to the third X-ray irradiation output at both ends, and the second X The line irradiation output is the largest at the center of the second type angle range and changes substantially without steps so as to be connected to the third X-ray irradiation output at both ends, and the third X-ray irradiation output is An X-ray CT apparatus characterized by being constant over a third type of angle range.

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