JP2000083946A - Method and device for correction projection and radiation tomography apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove a streak artifact by means of an object with a high radiation absorption rate by correcting the high frequency component of a projection at a specified corresponding position on the projection of several views concerning the image of the high radiation absorbing object of a tomographic image which is re-constituted from the projection. SOLUTION: When the projection PRj1 in the view j1 is the one in a figure, for example, and a sinogram SN, that is the projection of the object is positioned at Pi on an i-axis, correction is executed concerning a prescribed range (d) including the position Pi. A corresponding position is correctly specified concerning the position Pi by utilizing the sinogram. At the time of correction, signals are made to be the ones only with the high frequency component by processing the projection PRj1 by a proper high-pass filter, a signal strength in each high frequency component belonging to the range (d) among them is obtained and subtraction is executed in the strengths which correspond in position in the projection PRi1. Thus, the high frequency components of the range (d) are cancelled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a projection correction method and apparatus, and a radiation tomography apparatus.
In particular, the present invention relates to a projection correction method and apparatus for correcting high-frequency components of projection, and a radiation tomography apparatus including such a correction apparatus.

[0002]

2. Description of the Related Art As an example of a radiation tomography apparatus, for example, an X-ray computed tomography (CT) is used.
y) There is a device. In an X-ray CT apparatus, X-rays are used as radiation. An X-ray tube is used for X-ray generation.

An X-ray irradiating apparatus including an X-ray tube irradiates an X-ray beam having a width including an imaging range and a thickness in a direction perpendicular to the X-ray tube. The thickness of the X-ray beam can be changed by adjusting the aperture of an X-ray passing aperture (aperture) of a collimator, so that the thickness of a slice for imaging can be adjusted. Has become.

An X-ray detector is a multi-channel in which a large number (for example, about 1000) of X-ray detection elements are arranged in an array in the direction of the width of an X-ray beam.
e) an X-ray detector for detecting X-rays.

An X-ray irradiating / detecting device is rotated (scanned) around a subject, and projected images of the subject by X-rays in a plurality of view directions around the subject (projection: projection). ) Is measured, and a tomographic image is generated (reconstructed) based on the projections.

For example, an axial slice including both shoulders (a
When a slice having a long X-ray transmission path in the subject is photographed, as in the case of an xial slice, a high-frequency component associated with a decrease in X-ray intensity is included in the projection, thereby causing a streak artifact in a reconstructed image. A linear artifact called a "streak artifact" occurs. In order to suppress streak artifacts, data correction is performed to reduce high frequency components included in the projection.

[0007]

As described above, the data correction for reducing the high-frequency component has an undesirable side effect of decreasing the sharpness of the reconstructed image when the degree of reduction is increased. It was necessary to reduce high-frequency components to such an extent that they were not caused. Thus, for example,
There is a problem that strong streak artifacts derived from objects having a high X-ray absorptivity such as implanted metals and bones cannot be sufficiently suppressed.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a projection correction method and apparatus for removing streak artifacts due to an object having a high radiation absorptivity, and a correction apparatus such as this. To realize a radiation tomography apparatus provided with the same.

[0009]

(1) A first aspect of the present invention for solving the above-mentioned problem is that an image of a high-absorbing substance in a tomographic image reconstructed from a projection of a plurality of views by radiation is projected on the projection of the plurality of views. And correcting a high-frequency component of the projection at the corresponding position.

(2) According to a second aspect of the present invention, a corresponding position on the projection of the plurality of views is determined for each image of the high-absorbing material in the tomographic image reconstructed from the projection of the plurality of views by the radiation. A projection correction apparatus comprising: a position specifying unit for specifying; and a data correction unit for correcting a high-frequency component of the projection at the corresponding position.

(3) A third invention for solving the above-mentioned problems is a radiation tomography apparatus which collects projections of a plurality of views by radiation and generates a tomographic image based on the collected radiation projections. Position specifying means for respectively specifying corresponding positions on the projection of the plurality of views with respect to the image of the object, data correcting means for correcting high-frequency components of the projection at the corresponding positions, and a plurality of views corrected by the data correcting means. Image reconstruction means for reconstructing a tomographic image based on projection.

[0012] In any one of the first invention to the third invention, the specification of the corresponding position is performed based on a sinogram of the high radiation absorbing substance in the view channel space, so that the position specification is performed with high accuracy. It is preferable in that it is performed.

(Operation) In the present invention, the data correction for the high frequency component of the projection is performed at the position corresponding to the high radiation absorbing substance obtained from the tomographic image.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiment. FIG. 1 shows a block diagram of the X-ray CT apparatus. This apparatus is an example of an embodiment of the radiation tomography apparatus of the present invention. The configuration of the present apparatus shows an example of an embodiment relating to the apparatus of the present invention. An example of an embodiment of the method of the present invention is shown by the operation of the present apparatus.

As shown in FIG. 1, the present apparatus comprises a scanning gantry (gantry) 2 and a photographing table (table).
4 and an operation console 6. The scanning gantry 2 has an X-ray tube 20 as a radiation source. X-rays (not shown) emitted from the X-ray tube 20 are:
The collimator 22 shapes the beam into, for example, a fan-shaped X-ray beam, that is, a fan beam, and irradiates the detector array 24 with the beam.
The detector array 24 has a plurality of X-ray detection elements arranged in an array in the direction of the width of the fan-shaped X-ray beam. The configuration of the detector array 24 will be described later. X-ray tube 20, collimator 22, and detector array 2
4 constitutes an X-ray irradiation / detection device. The X-ray irradiation / detection device will be described later.

A data collection unit 26 is connected to the detector array 24. The data collection unit 26 includes the detector array 24
The detection data of the individual X-ray detection elements are collected.

The irradiation of X-rays from the X-ray tube 20 is controlled by an X-ray controller (controller) 28. The illustration of the connection relationship between the X-ray tube 20 and the X-ray controller 28 is omitted. The collimator 22 is controlled by a collimator controller 30. The illustration of the connection relationship between the collimator 22 and the collimator controller 30 is omitted.

The above-described X-ray tube 20 to collimator controller 30 are mounted on the rotating unit 32 of the scanning gantry 2. The rotation of the rotation unit 32 is controlled by a rotation controller 34. The illustration of the connection relationship between the rotation unit 32 and the rotation controller 34 is omitted.

The imaging table 4 carries a subject (not shown) into and out of the X-ray irradiation space of the scanning gantry 2. The relationship between the subject and the X-ray irradiation space will be described later.

The operation console 6 has a central processing unit 60. The central processing unit 60 is constituted by, for example, a computer. The central processing unit 60 is an example of an embodiment of the projection correction device of the present invention. The central processing unit 60 is an example of an embodiment of a position specifying unit according to the present invention. It is also an example of an embodiment of the data correction means in the present invention. Also, this is an example of an embodiment of an image reconstructing unit according to the present invention.

The central processing unit 60 is connected with a control interface (interface) 62. The scanning gantry 2 and the imaging table 4 are connected to the control interface 62.

The central processing unit 60 has a control interface 6
2, the scanning gantry 2 and the imaging table 4 are controlled. The data acquisition unit 26, X-ray controller 28, collimator controller 30, and rotation controller 34 in the scanning gantry 2 are controlled through a control interface 62. It should be noted that illustration of individual connections between these units and the control interface 62 is omitted.

A data acquisition buffer 64 is also connected to the central processing unit 60. Data collection buffer 6
4 is connected to the data collection unit 26 of the scanning gantry 2. The data collected by the data collection unit 26 is input to the data collection buffer 64. Data collection buffer 6
4 temporarily stores the input data.

The central processing unit 60 performs image reconstruction based on data of a plurality of views collected through the data collection buffer 64. Image reconstruction includes, for example, filtered back projection.
For example, a k projection method is used. The storage device 66 is also connected to the central processing unit 60. The storage device 66 stores various data, reconstructed images, programs, and the like.

The central processing unit 60 also includes a display device 6
8 and the operating device 70 are connected to each other. The display device 68 displays a reconstructed image and other information output from the central processing unit 60. Operation device 7
Numeral 0 is operated by the operator to input various instructions and information to the central processing unit 60.

FIG. 2 shows a schematic configuration of the detector array 24. The detector array 24 includes a large number of X-ray detection elements 24.
This is a multi-channel X-ray detector in which (i) is arranged. The large number of X-ray detection elements 24 (i)
An X-ray incident surface curved in a cylindrical concave shape is formed. i is a channel number, for example, i = 1 to 1000.

The X-ray detecting element 24 (i) is composed of, for example, a combination of a scintillator and a photodiode. The present invention is not limited to this, and may be, for example, a semiconductor X-ray detection element using cadmium tellurium (CdTe) or the like, or an ionization box type X-ray detection element using xenon (Xe) gas.

FIG. 3 shows the relationship among the X-ray tube 20, collimator 22, and detector array 24 in the X-ray irradiation / detection device. 3A is a diagram illustrating a state viewed from the front, and FIG. 3B is a diagram illustrating a state viewed from the side. As shown in the figure, the X-rays radiated from the X-ray tube 20 are shaped into a fan-shaped X-ray beam 40 by a collimator 22 and irradiated to a detector array 24.

FIG. 3A shows a fan-shaped X-ray beam 40.
, Ie, the width of the X-ray beam 40. The width direction of the X-ray beam 40 matches the arrangement direction of the channels in the detector array 24. In (b), the X-ray beam 40
Shows the thickness of

By making the body axis cross the fan surface of such an X-ray beam 40, for example, as shown in FIG.
Is placed in the X-ray irradiation space. X
A projection image of the subject 8 sliced by the line beam 40 is projected on the detector array 24. The thickness t of the X-ray beam 40 irradiating the subject 8 is set by adjusting the aperture of the aperture of the collimator 22.

The operation of the present apparatus will be described. The operator sets desired photographing conditions through the operation device 70. Next, the subject 8 is scanned by the X-ray irradiation / detection device under the control of the central processing unit 60. As a result, a projection of, for example, 1000 views of the subject 8 is collected in the data collection buffer 64.

The projection of a plurality of views collected in the data collection buffer 64 forms a two-dimensional view channel space u as shown in FIG. 5, for example. In the view channel space u, the horizontal axis i represents a channel number, and the vertical axis j represents a view number. Also, a plurality of projection data (projection) connected in the i direction
data) PRij indicates the projection PRj in the j-view. The projection PRj is an example of an embodiment of the projection in the present invention.

Based on such a projection PRj, the central processing unit 60 performs image reconstruction by, for example, a filtered back projection method or the like, and generates a tomographic image. Thereby, for example, a tomographic image of the subject 8 as shown in FIG. 6 is obtained.

When an object 90 having a high X-ray absorptivity, such as an embedded metal, is present inside the subject 8, a streak artifact 92 due to the object 90 is included in the reconstructed image. The object 90 having a high X-ray absorptivity is not limited to an embedded metal, but may be a bone, a calculus, or the like. Object 9 with high X-ray absorption
Numeral 0 is an example of an embodiment of the radiation-absorbent according to the present invention.

When such streak artifacts occur, in the present apparatus, the streak artifacts are removed by the central processing unit 60 as follows. FIG.
FIG. 7 shows a flow chart of a procedure for removing streak artifacts. As shown in the figure, in step (702), a binary image of the tomographic image shown in FIG. 6 is generated.

The binary image has an appropriate CT number (num)
ber) (for example, 1000) as a threshold value, and a pixel (pixel) having a CT number higher than the threshold value is set to 1,
The other pixels are generated as 0. Thereby, for example, as shown in FIG.
A binary image containing only zeros is obtained.

Next, in step 704, a sinogram (si
nogram). To find the sinogram,
As schematically shown in FIG. 9, the same geometry (geo) as that of the X-ray irradiation / detection device at the time of scanning the subject 8 is obtained for the above binary image by data processing inside the central processing unit.
metry) to obtain a plurality of projection images of the same view as at the time of scanning.

As a result, the projection position Pi of the object 90 on the i-axis is determined for each view. For example, as shown in FIG.
Of the projection position Pi, that is, the sinogram SN is obtained.

When there are a plurality of objects 90, a plurality of sinograms SN are obtained correspondingly. Hereinafter, a case where the sinogram SN is single will be described, but the same applies to a case where a plurality of sinograms are used.

Next, in step 706, the projection is corrected. The correction of the projection is performed by removing or reducing high-frequency components included in the projection. At this time, the portion to be corrected is sinogram SN
Specify based on.

That is, if the projection PRj1 in the view j1 is as shown in FIG. 11, for example, and the projection of the sinogram SN, that is, the projection of the object 90 is at the position Pi on the i-axis, the position Pi Is corrected for a predetermined range d including.
The range d is appropriately determined. The position Pi is an example of an embodiment of the “corresponding position on the projection” in the present invention. By using the sinogram, the corresponding position can be specified accurately.

In the correction, by processing the projection PRj1 with an appropriate high-pass filter, a signal having only high-frequency components as shown in FIG. 12 is obtained. Determine the signal strength and calculate
In the projection PRj1, those whose positions correspond to each other are subtracted. As a result, high frequency components in the range d are canceled (removed).

Alternatively, instead of completely canceling each other, each high-frequency component is subtracted by multiplying it by an appropriate weighting factor, or
Each high-frequency component may be reduced at an appropriate ratio. The use of the weight coefficient or the ratio coefficient in this manner is preferable in that the strength of the correction is appropriately adjusted.

Such correction is performed for the projections PRj of all views, and high-frequency components in the portion corresponding to the sinogram SN are removed or reduced from the projections of all views.

Next, in step 708, image reconstruction is performed again based on the corrected projection.
Thus, as shown in FIG. 13, for example, it is possible to obtain a tomographic image including an image of the object 90 having a high X-ray absorptivity, but having no streak artifact.
It is needless to say that the object 90 having a high X-ray absorption rate is not limited to metal, but may be bone or the like.

Since the high-frequency components of the portion other than the range d remain unchanged even in the corrected projection, the sharpness of the image is not impaired even if the above correction is performed. For this reason, the projection correction can be made sufficiently strong without considering the adverse effect on the sharpness of the image. Therefore, streak artifacts can be sufficiently reduced without impairing the sharpness of the image.

Although the example using X-rays as radiation has been described above, the radiation is not limited to X-rays, but may be other types of radiation such as γ-rays. However,
At present, X-rays are preferred because practical means for generating, detecting, controlling, and the like are the most substantial.

[0048]

As described above in detail, according to the present invention, a projection correction method and apparatus for removing streak artifacts due to an object having a high radiation absorption rate, and a radiation tomography provided with such a correction apparatus The device can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram of a device according to an example of an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a detector array in an apparatus according to an embodiment of the present invention.

FIG. 3 is a schematic configuration diagram of an X-ray irradiation / detection device in an example of an embodiment of the present invention.

FIG. 4 is a schematic configuration diagram of an X-ray irradiation / detection device in the device according to an embodiment of the present invention.

FIG. 5 is a schematic diagram of a view channel space in the apparatus according to an embodiment of the present invention;

FIG. 6 is a schematic diagram of a tomographic image taken by an apparatus according to an embodiment of the present invention;

FIG. 7 is a flowchart showing an operation of the apparatus according to the embodiment of the present invention;

FIG. 8 shows an example of an image generated by an apparatus according to an embodiment of the present invention.
It is a schematic diagram of a value image.

FIG. 9 is a schematic diagram of a sinogram generation by the apparatus according to the embodiment of the present invention;

FIG. 10 is a schematic diagram of a sinogram generated by an apparatus according to an example of an embodiment of the present invention.

FIG. 11 is a schematic diagram of a projection of a subject obtained by an apparatus according to an embodiment of the present invention.

FIG. 12 is a schematic diagram of a high-frequency component of the projection of the subject obtained by the apparatus according to the embodiment of the present invention;

FIG. 13 is a schematic diagram of a tomographic image generated by an apparatus according to an embodiment of the present invention.

[Explanation of symbols]

 2 Scanning Gantry 20 X-ray Tube 22 Collimator 24 Detector Array 26 Data Collection Unit 28 X-ray Controller 30 Collimator Controller 32 Rotation Unit 34 Rotation Controller 4 Imaging Table 6 Operation Console 60 Central Processing Unit 62 Control Interface 64 Data Collection Buffer 66 Storage Device 68 display device 70 operating device 40 X-ray beam 8 subject 24 (i) X-ray detecting element

Claims (3)

[Claims] 1. A corresponding position on a projection of a plurality of views is specified for an image of a radiation-absorbing material in a tomographic image reconstructed from a projection of a plurality of views by radiation, and a high-frequency component of the projection at the corresponding position is determined. A projection correction method, comprising: correcting. 2. A position specifying means for specifying a corresponding position on the projection of the plurality of views with respect to an image of a high-absorbing substance in a tomographic image reconstructed from projections of a plurality of views by radiation, and the projection at the corresponding position. And a data correction means for correcting the high-frequency component of the projection. 3. A radiation tomography apparatus for collecting projections of a plurality of views by radiation and generating a tomographic image based on the collected projections, wherein correspondence of an image of a high-absorbing substance in the tomographic images on the projections of the plurality of views is provided. Position specifying means for specifying respective positions; data correction means for correcting high-frequency components of the projection at the corresponding positions; image reconstruction for reconstructing a tomographic image based on the projections of a plurality of views corrected by the data correction means And radiation means.