JP2000033085A - X-ray computerized tomograph - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of an artifact (false image) in a CT (computerized tomography) images. SOLUTION: In this X-ray computerized tomograph, the relation of the fluctuation amount of X-ray detection data generated by changing the focus position of an X-ray tube 2 to an X-ray detector 3 by the rotation of the X-ray tube 2 or the inclination of a gantry 6, the rotation angle of the X-ray tube and the inclination angle of the gantry is stored in a correction data table 20 as correction data beforehand. At the time of executing a signal processing for image reconstitution to the X-ray detection data obtained from the X-ray detector 3 accompanying the irradiation of a fan beam FB, a correcting operation for eliminating the fluctuation of the X-ray detection data due to the change of the focus position of the X-ray tube by using the correction data in the correction data table 20 is performed. Thus, error fluctuation causing an artifact is removed from the X-ray detection data.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a computer tomographic image (CT image) based on X-ray detection data output from a multi-channel type X-ray detector when a subject is irradiated with a fan-shaped X-ray beam. The present invention relates to an X-ray CT apparatus that finally obtains an image, and more particularly to a technique for preventing an artifact (false image) from appearing in a CT image.

[0002]

2. Description of the Related Art Conventionally, an X-ray CT apparatus used in a hospital or the like is, as shown in FIG. An X-ray tube 61 for irradiating the FB and a number of X-ray detection elements for detecting transmitted X-rays corresponding to the amount determined by the channel collimator are provided by the fan beam F.
The multi-channel X-ray detector 62 arranged along the spread of B, the X-ray tube 61 and the X-ray detector 62 while maintaining a state in which the object M on the top plate 60 is opposed to each other. In addition to the gantry 63 provided so as to be able to rotate around the body axis Z of the subject M, the X-ray tube 61
While turning around the body axis (the axis perpendicular to the paper surface) Z of the subject M in the directions indicated by the arrows RA and RB, the fan beam F
An image reconstruction unit (not shown) that performs image signal processing on the X-ray detection data output from the X-ray detector 62 in accordance with the irradiation of B to perform image reconstruction for a CT image is provided. . Normally, the gantry 63 does not tilt the fan beam F
B is irradiated at right angles to the body axis Z of the subject M to perform X-ray imaging. However, depending on the situation, the gantry 63 is inclined to irradiate the fan beam FB obliquely to the body axis Z of the subject M. Sometimes X-rays are taken.

The multi-channel X-ray detector 62 of the conventional device shown in FIG. 11 is a solid-state multi-channel X-ray detector.
It is a line detector. That is, the X-ray detector 62 is
As shown in (1), the X-ray detecting element is the solid-state detecting element 64, and the channel collimator 65 is configured by an X-ray separating plate 65a disposed at each channel boundary position on the front side of the solid-state detecting element.

[0004]

However, in the case of the above-mentioned conventional X-ray CT apparatus, as will be described in detail later, the focal point of the X-ray tube 61 is displaced as the gantry 63 rotates or tilts. For this reason, there is a problem that an arc-shaped artifact appears in the CT image.

In view of the above circumstances, an object of the present invention is to provide an X-ray CT apparatus capable of preventing the appearance of artifacts in a CT image.

[0006]

In order to achieve the above object, an X-ray CT apparatus according to the first aspect of the present invention comprises: an X-ray tube for irradiating a subject on a top plate with a fan-shaped X-ray beam; A multi-channel type X-ray detector in which a number of X-ray detection elements each detecting a transmission X-ray corresponding to an area determined by a channel collimator are arranged along the spread of a fan-shaped X-ray beam; A gantry is provided so that the X-ray detector can be rotated around the body axis of the subject while maintaining a state where the X-ray detector faces the subject on the top plate, and the X-ray tube is covered. The image signal processing is performed on the X-ray detection data output from the X-ray detector as the fan-shaped X-ray beam is irradiated while rotating around the body axis of the sample.
In an X-ray CT apparatus provided with image reconstruction means for performing image reconstruction for a T image, the focal position of the X-ray tube relative to the X-ray detector changes when the X-ray tube rotates around the body axis of the subject. And a rotation fluctuation characteristic storage unit for storing in advance the relationship between the amount of fluctuation of the X-ray detection data and the rotation angle of the X-ray tube, the image reconstruction means being stored in the rotation fluctuation characteristic storage unit. A correction operation for eliminating error fluctuation of X-ray detection data caused by a change in the focal position of the X-ray tube due to the rotation of the X-ray tube according to the relationship between the amount of fluctuation of the X-ray detection data and the rotation angle of the X-ray tube. It is configured to perform the processing.

In order to achieve the above object, an X-ray CT apparatus according to a second aspect of the present invention is determined by an X-ray tube for irradiating a subject on a top plate with a fan-shaped X-ray beam and a channel collimator. A multi-channel X-ray detector in which a number of X-ray detection elements for detecting the transmitted X-rays of the respective regions are arranged along the spread of the fan-shaped X-ray beam, an X-ray tube and an X-ray detector A gantry equipped so as to be able to rotate around the body axis of the subject while maintaining a state of opposition sandwiching the subject on the top plate, and the X-ray tube is mounted on the body axis of the subject. An image reconstruction means for performing image signal processing on X-ray detection data output from the X-ray detector in conjunction with irradiating a fan-shaped X-ray beam while rotating around to perform CT image reconstruction; X-ray C
In the T apparatus, gantry tilting means for tilting the gantry so that a fan-shaped X-ray beam from the X-ray tube is irradiated obliquely to the subject, and a focal position of the X-ray tube with respect to the X-ray detector when the gantry is tilted And a tilt variation characteristic storage unit for storing in advance a relationship between a variation amount of the X-ray detection data and a tilt angle of the gantry caused by the change of the image data, and the image reconstruction unit stores the relationship in the tilt variation characteristic storage unit. A correction operation for eliminating an error fluctuation of the X-ray detection data caused by a change in the focal position of the X-ray tube due to the inclination of the gantry according to the relationship between the fluctuation amount of the X-ray detection data and the inclination angle of the gantry is performed by image signal processing. It is configured to do so.

In order to achieve the above object, an X-ray CT apparatus according to a third aspect of the present invention is determined by an X-ray tube for irradiating a subject on a top plate with a fan-shaped X-ray beam and a channel collimator. A multi-channel X-ray detector in which a number of X-ray detection elements for detecting the transmitted X-rays of the respective regions are arranged along the spread of the fan-shaped X-ray beam, an X-ray tube and an X-ray detector A gantry equipped so as to be able to rotate around the body axis of the subject while maintaining a state of opposition sandwiching the subject on the top plate, and the X-ray tube is mounted on the body axis of the subject. An image reconstruction means for performing image signal processing on X-ray detection data output from the X-ray detector in conjunction with irradiating a fan-shaped X-ray beam while rotating around to perform CT image reconstruction; X-ray C
In the T apparatus, gantry tilting means for tilting the gantry so that a fan-shaped X-ray beam from the X-ray tube is obliquely irradiated onto the subject, and rotation of the X-ray tube around the body axis of the subject or tilting of the gantry Tilt variation that stores in advance the relationship between the amount of variation in X-ray detection data and the rotation angle of the X-ray tube and the tilt angle of the gantry caused by a change in the focal position of the X-ray tube with respect to the X-ray detector when performing The image reconstructing means includes a characteristic storing means, and the image reconstructing means performs X-ray imaging in accordance with the relationship between the amount of fluctuation of the X-ray detection data stored in the rotational inclination fluctuation characteristic storing means and the rotation angle of the X-ray tube and the inclination angle of the gantry. The image signal processing is performed to perform a correction operation for eliminating an error fluctuation of X-ray detection data caused by a change in the focal position of the X-ray tube due to the rotation of the X-ray tube and the inclination of the gantry. .

The X-ray CT apparatus according to the invention of claim 4 is
4. The X-ray CT apparatus according to claim 1, wherein the multi-channel X-ray detector has a configuration in which the X-ray detection element is a solid-state detection element and the channel collimator is an X-ray incident side of the solid-state detection element. Is a solid-element type multi-channel X-ray detector comprising an X-ray partition plate disposed at each channel boundary position.

[Operation] Next, the effect of preventing artifacts in X-ray photography by the X-ray CT apparatus of the present invention will be described. In the X-ray CT apparatus according to the first aspect, at the time of X-ray imaging, the X-ray tube rotates around the body axis of the subject and forms a fan-shaped X-ray.
At the same time as irradiating the X-ray beam, X-ray detection data output from the X-ray detector accompanying the irradiation of the fan-shaped X-ray beam is collected. Then, in the image signal processing performed on the X-ray detection data collected by the image reconstructing means for the image reconstruction for the CT image, the amount of fluctuation of the X-ray detection data stored in advance in the rotation fluctuation characteristic storing means is determined. By performing a correction operation in accordance with the relationship between the rotation angles of the X-ray tube, fluctuations in X-ray detection data caused by changes in the focal position of the X-ray tube due to rotation of the X-ray tube are eliminated.

That is, during one rotation of the X-ray tube, the focal position of the X-ray tube with respect to the X-ray detector changes every moment according to the rotation angle of the X-ray tube. The amount of transmitted X-ray incident on each X-ray detection element changes every moment according to the change of the focal position of the X-ray tube.
In addition to the true fluctuation due to the X-ray absorption, an error fluctuation due to a change in the focal position due to the rotation of the X-ray tube is added. The error fluctuation in the X-ray detection data causes an artifact. On the other hand, the X-ray CT apparatus according to claim 1 performs a correction operation in accordance with a relationship between a fluctuation amount of the X-ray detection data stored in advance and a rotation angle of the X-ray tube, thereby obtaining an X-ray detection data causing an artifact. Eliminates error fluctuations.

Further, in the X-ray CT apparatus according to the second aspect, the gantry can be tilted by the gantry tilting means to perform X-ray imaging. The image reconstructing means of the X-ray CT apparatus according to claim 2, wherein the image signal processing of the X-ray detection data, the variation amount of the X-ray detection data stored in advance in the tilt variation characteristic storage means and the tilt angle of the gantry By performing the correction calculation according to the relationship, the fluctuation of the X-ray detection data caused by the change of the focal position of the X-ray tube due to the inclination of the gantry is eliminated.
The focal position of the X-ray tube with respect to the X-ray detector also changes due to the inclination of the gantry, and the amount of transmitted X-ray incident on each X-ray detection element changes. In addition to the true variation due to the above, an error variation due to a change in the focal position of the X-ray tube due to the inclination of the gantry is added, and this error variation also causes an artifact. On the other hand, in the X-ray CT apparatus according to the second aspect, the correction operation is performed in accordance with the relationship between the variation amount of the X-ray detection data stored in advance and the inclination angle of the gantry, so that the X-ray detection data causing an artifact can be obtained. Eliminate error fluctuations.

The X-ray CT apparatus according to the third aspect can also perform imaging by tilting the gantry by the gantry tilting means. The image reconstruction means of the X-ray CT apparatus according to claim 3 is
At the time of image signal processing of the X-ray detection data, a correction operation is performed in accordance with the relationship between the fluctuation amount of the X-ray detection data stored in advance in the rotation inclination fluctuation characteristic storage means and the rotation angle of the X-ray tube and the gantry inclination angle. This eliminates fluctuations in X-ray detection data caused by changes in the focal position of the X-ray tube due to rotation of the X-ray tube and inclination of the gantry. That is, during one rotation of the X-ray tube, the focal position of the X-ray tube with respect to the X-ray detector changes every moment according to both the rotation angle of the X-ray tube and the tilt angle of the gantry, and the focal position of the X-ray tube is fluctuate. This X
Since the amount of transmitted X-ray incident on each X-ray detection element also changes according to the change in the focal position of the X-ray tube, in addition to the true change due to the X-ray absorption of the imaging target site as the change in the X-ray detection data,
An error fluctuation due to a change in the focal position due to the rotation of the X-ray tube is added. This error variation causes an artifact. On the other hand, in the X-ray CT apparatus according to the third aspect, the correction operation is performed in accordance with the relationship between the fluctuation amount of the X-ray detection data stored in advance and the rotation angle of the X-ray tube and the tilt angle of the gantry, thereby reducing artifacts. Cause X
Eliminates error fluctuations in line detection data.

Further, in the X-ray CT apparatus according to the fourth aspect, a solid-state multi-channel X-ray detector in which the X-ray detecting element is a solid-state detecting element is used. In the case of a solid-state X-ray detector, an X-ray separator plate constituting a channel collimator is installed at each channel boundary position on the X-ray incident side of the solid-state device, and a solid-state detector is provided at the bottom of the X-ray separator plate. Due to the position of the detection element, a change in the X-ray incident amount greatly changes due to a change in the focal position of the X-ray tube due to rotation of the X-ray tube or inclination of the gantry, and significant error fluctuation occurs in the X-ray detection data. . However, this remarkable error fluctuation is also removed by the correction operation performed in the image signal processing for the X-ray detection data.

[0015]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an X-ray C according to the embodiment.
FIG. 2 is a block diagram showing the overall configuration of the T apparatus, FIG. 2 is a schematic view showing the main configuration of a solid-state multi-channel X-ray detector of the embodiment apparatus, and FIG. It is a front view. The X-ray CT apparatus shown in FIG. 1 is determined by an X-ray tube 2 for irradiating a subject M on a top plate 1 with a fan beam (fan-shaped X-ray beam) FB and a channel collimator 4 as shown in FIG. A solid-state multi-channel X-ray detector 3 in which a large number of solid-state detection elements 5 for X-ray detection for respectively detecting the transmitted X-rays of the possession are arranged along the spread of the fan beam FB; A gantry 6 equipped with the tube 2 and the X-ray detector 3 so as to be rotatable around the body axis Z of the subject M while maintaining a state in which the tube 2 and the X-ray detector 3 are opposed to each other with the subject M interposed therebetween. Have.

At the time of X-ray photography, the tabletop 1 enters or leaves the tunnel 6a of the gantry 6 with the subject M placed thereon.
The top 1 is moved by the control of the top drive 7. The X-ray tube 2 and the X-ray detector 3 are arranged inside the gantry 6, and the X-ray tube 2 and the X-ray detector 3 are connected by a rotating ring RG. The X-ray tube 2 and the X-ray detector 3 rotate around the body axis Z of the subject M in the direction indicated by the arrows RA and RB while maintaining the facing arrangement state in accordance with the rotation of the rotating ring RG by 8. . Further, the X-ray tube 2 irradiates the fan M with the fan beam FB according to the set irradiation conditions such as the tube voltage and the tube current under the control of the X-ray irradiation control unit 9 including the high voltage generator during rotation. Has also become.

On the other hand, in the X-ray detector 3 for detecting transmitted X-rays from the subject M, as shown in FIG.
The X-ray separating plate 4a disposed at each channel boundary position on the X-ray incidence side of the channel collimator 4
Is configured. The incidence of unnecessary scattered X-rays is blocked by these X-ray partitioning plates 4a. Usually, the number of channels of the X-ray detector 3 is around 1000. X-ray detector 3
The solid-state detection element 5 is not limited to a specific element. For example, an element in which a scintillator layer that converts transmitted X-rays into light and a light detection device that detects the intensity of light converted by the scintillator layer is used. Is exemplified. Normally, an X-ray detector 3 is configured by connecting a required number of substrates on which a plurality of solid-state detection elements 5 arranged along the spread of the fan beam FB are formed. Also,
The solid-state detection elements 5 are not necessarily limited to a single-row configuration, and may have a multi-row configuration in which the solid-state detection elements are also adjacent in the direction of the body axis Z of the subject M. X-ray detection data obtained by each solid-state detection element 5 is collected by the data collection unit 10.

In the case of the apparatus of the embodiment, as shown in FIG. 3, the gantry 6 is normally upright without tilting, and is irradiated with the fan beam FB from the direction perpendicular to the body axis Z of the subject M. However, by tilting the gantry 6 in a direction in which the gantry 6 is tilted toward the head side or the foot side with the vicinity of the imaging center Ma as a fulcrum, the fan beam FB can be irradiated from an oblique direction with respect to the body axis Z of the subject M. Is also configured. If the gantry 6 is tilted toward the head, the fan beam FB is tilted at the angle θa.
When the gantry 6 is tilted toward the foot, the fan beam FB is irradiated obliquely by the tilt angle θb. The inclination of the gantry 6 is controlled by the control of the gantry inclination drive unit 11. Note that the controls by the top board driving unit 7, the imaging system rotation driving unit 8, the X-ray irradiation control unit 9, and the gantry tilt driving unit 11 are all controlled by the input operation with the keyboard 12 and the mouse 13. This is performed according to a command signal sent from the unit 14.

During the execution of X-ray imaging, X-ray detection data output from the X-ray detector 3 accompanying the irradiation of the fan beam FB from the rotating X-ray tube 2 is collected by the data collection unit 10. The data is sent to an AD converter 15 for digitizing the data. After the AD conversion unit 15, a detection data memory 16 for storing X-ray detection data and an image reconstruction unit 17
And a CT image memory 18 are provided. The X-ray detection data collected by the data collection unit 10 is temporarily stored in the detection data memory 16 and then read out in a timely manner. Image reconstruction for the image is performed. further,
A CT image signal relating to a section designated by an input operation or the like using the keyboard 12 or the mouse 13 is stored in a CT image memory 18.
And a CT image of the designated cross section is displayed on the screen of the display monitor 19. In addition to the image display by the display monitor 19, a CT image signal is printed on a sheet by an image printing recording unit (not shown) called a film imager or the like and output as an X-ray photograph, or a memory disk (not shown). )
Alternatively, the configuration may be such that the information is stored and stored.

The X-ray CT apparatus of the embodiment has the following salient features. That is, the device of the embodiment is X
The amount of change in X-ray detection data caused by the change of the focal position of the X-ray tube 2 with respect to the X-ray detector 3 due to the rotation of the X-ray tube 2 or the inclination of the gantry 6, the rotation angle θr of the X-ray tube 2 and the A correction data table (rotational inclination fluctuation characteristic storage means) 20 in which the relationship between the inclination angles θa and θb (rotational inclination fluctuation characteristic) is stored in advance, and X-ray detection data from the correction data stored in the correction data table 20. The image reconstruction unit 17 includes a correction data reading unit 21 that reads correction data necessary for the correction calculation, and the image reconstructing unit 17 controls the rotation of the X-ray tube 2 and the gantry 6 according to the rotation inclination variation characteristics stored in the correction data table 20. Of the X-ray tube 2 caused by the change in the focal position of the X-ray tube due to the inclination of the X-ray tube. That. Hereinafter, these salient features will be described in detail.

As shown in FIG. 4, the gravity Gf due to its own weight always acts on the X-ray tube 2 irrespective of the presence or absence of rotation, and the centrifugal force Hf generated by the rotation during rotation.
Also works. Since the directions of the gravitational force Gf and the centrifugal force Hf acting on the X-ray tube 2 greatly change in accordance with the rotation angle θr of the X-ray tube 2, the resultant force Tf of all the forces acting on the X-ray tube 2 is X-ray. It changes greatly according to the rotation angle θr of the tube 2. Thus, during the execution of X-ray imaging, the X-ray tube 2
Force Tf acting on the X-ray tube 2 according to the rotation angle θr of
Greatly changes the focal position of the X-ray tube 2 with respect to the X-ray detector 3.

When the gantry 6 is tilted, FIG.
As shown in the figure, the direction of the gravity Gf acting on the X-ray tube 2 changes according to the inclination angles θa and θb. In particular, since the target of the X-ray tube 2 is very heavy,
Only by changing the direction of the gravitational force Gf acting on the X-ray tube 2, the positional relationship between the target and the filament changes, and the focal position of the X-ray tube 2 changes. Of course, when the X-ray tube 2 is rotated, the gravity Gf and the centrifugal force Hf applied to the X-ray tube 2
Changes according to the rotation angle θr of the X-ray tube 2. As a result, during X-ray imaging with the gantry 6 tilted, the combined force Tf of all the forces acting on the rotating X-ray tube 2
Greatly changes in accordance with both the rotation angle θr and the inclination angles θa and θb of the X-ray tube 2. The focal position of the X-ray tube 2 with respect to the detector 3 also changes accordingly.

As described above, when the focal position of the X-ray tube 2 with respect to the X-ray detector 3 changes, (the intensity of) the X-ray detection data changes. That is, as shown in FIG.
Even if the focal point of the X-ray tube 2 is set at the position Na so that the entire share of the fan beam FB is just incident on the solid-state detecting element 5, if the focal point changes to the position Nb during execution of X-ray imaging, the partition The shadow of the fan beam FB (shaded area in FIG. 6) is formed by being blocked by the plate 6a, and a part of the fan beam FB does not reach the solid-state detection element 5 and cannot enter. The intensity of the X-ray detection data is reduced by a part of the fan beam FB that cannot be incident. That is, the X-ray tube 2
During one rotation, the focal position of the X-ray tube 2 with respect to the X-ray detector 3 changes every moment with the change of the rotation angle θr. In addition to the fluctuations, error fluctuations due to changes in the focal position of the X-ray tube are added. If the image reconstruction is performed without eliminating the error fluctuation of the X-ray detection data, the final CT
Arc-shaped artifacts appear in the image. for that reason,
A correction operation for eliminating error fluctuation of X-ray detection data based on the correction data stored in the correction data table 20 is required.

However, since the fluctuation rate is the same for the X-ray detection data in which the rotation angle of the X-ray tube 2 is the same during each rotation, it is sufficient to store the rotation inclination fluctuation characteristic for one rotation. Of course, if the inclination angles θa and θb are different, the rotation inclination fluctuation characteristic changes. Therefore, for some inclination angles θa and θb, one rotation of the X-ray tube 2 (rotation angle θr = 0 ° or more).
360 °) is stored. In the case of the embodiment, as shown in FIGS. 7A to 7C, the inclination angle θa = 25.
Correction data for one rotation of the X-ray tube 2 (rotation angle θr = 0 ° to 360 °) for three angles of inclination angle θa = θb = 0 ° (non-inclination state) and inclination angle θb = 25 °. This is stored in the correction data table 20 in advance.

The correction data shown in FIGS. 7A to 7C is obtained by obtaining, for each channel, X-ray detection data in a state where an imaging part is empty, and averaging the X-ray detection data for one rotation for each channel. The ratio α of the X-ray detection data with respect to the average value is calculated for each channel, and this is stored as correction data. If the X-ray detection data is the same as the average,
If the correction data is “α = 1” and the X-ray detection data is 90% of the average value, the correction data is “α = 0.9” and the X-ray detection data is 110% of the average value. For example, the correction data is “α = 1.1”. Also, FIG.
The correction data shown in (c) may be stored in the form of a function, for example, 36 ° out of the rotation angle θr = 0 ° to 360 °.
If the correction data is skipped and stored only for each rotation angle, it is practical because the storage capacity can be reduced. If the correction data is skipped and stored only for every tenth channel, for example, without storing the correction data for all the channels, the storage capacity can be reduced, which is practical.

The reading of the correction data from the correction table 20 is performed by the correction data reading unit 21.
The inclination angle θ of the gantry 6 from the gantry tilt drive unit 11 to the correction data readout unit 21 via the image reconstruction unit 17
When the data indicating “a” and “θb” is sent, the correction data reading unit 21 sets the rotation angle θr of the corresponding tilt angle to 0 ° to 36 °.
The configuration is such that the correction data of 0 ° is read for each required channel and transmitted to the image reconstruction unit 17. Of course, the configuration may be such that the correction data for all the channels is read at once. In the case of the apparatus according to the embodiment, the inclination angles θa, θ
When b is other than 0 ° and 25 °, the inclination angles θa and θb
Interpolation calculation (interpolation calculation by linear interpolation or least square method) using correction data of an appropriate angle among 0 ° and 25 °
And the rotation angle θr of the corresponding inclination angle is 0 ° to 360 °
The correction data is obtained and transmitted to the image reconstruction unit 17.

Further, when the correction data of the rotation angle or the channel is skipped and stored, the correction data of the closest rotation angle or the channel may be read instead, but an appropriate one from the stored correction data may be used. It is preferable to read out the data, perform an interpolation operation to obtain appropriate correction data, and then send the data to the image reconstructing unit 17 because an accurate correction operation can be performed.

The image reconstruction unit 17 performs a correction operation using the correction data received from the correction data reading unit 21. That is, since the correction data is a ratio α to the average value, each actual X obtained in each channel is obtained.
If the line detection data is multiplied (divided) by the ratio α of the corresponding correction data, the fluctuation of the X-ray detection data caused by the change of the focal position of the X-ray tube 2 due to the rotation of the X-ray tube 2 and the inclination of the gantry 6 Will be erased. However, in the image signal processing process in the image reconstruction unit 17, logarithmic calculation processing is usually performed on the X-ray detection data. In this case, the correction calculation involves subtracting the logarithmic value of the ratio α.

As described above, in the X-ray CT apparatus according to the embodiment, the error fluctuation included in the X-ray detection data due to both the rotation of the X-ray tube 2 and the inclination of the gantry 6 is stored in advance. Since the correction is performed according to the relationship between the fluctuation amount of the X-ray detection data and the rotation angle of the X-ray tube and the inclination angle of the gantry, artifacts do not appear in the finally obtained CT image.

Next, the operation of the apparatus for executing X-ray photography by the embodiment apparatus having the above-described configuration will be described with reference to the drawings. FIG. 8 is a flowchart illustrating the flow of the X-ray imaging operation performed by the apparatus according to the embodiment. After placing the subject M on the top 1 and setting it at the imaging position of the tunnel 6a of the gantry 6, the gantry 6 is tilted at an angle θa = 25.
° so that it is inclined.

[Step S1] When X-ray imaging is started, the X-ray tube 2 and the X-ray detector 3 start rotating around the body axis Z of the subject M.

[Step S2] When the rotations of the X-ray tube 2 and the X-ray detector 3 enter a stable state, the X-ray tube 2 radiates the fan beam FB to the subject M, and simultaneously transmits the fan beam FB from the subject M. X-rays are detected by the X-ray detector 3 and sent out as X-ray detection data.

[Step S3] X-ray detection data output from the X-ray detector 3 is collected and stored in the detection data memory 16
Are stored one after another.

[Step S4] When a command to read out the correction data at the inclination angle θa = 25 ° of the gantry 6 is sent from the image reconstruction unit 17 to the correction data reading unit 21, the correction data reading unit 21 immediately proceeds to FIG. The correction data of (a) is read out and sent to the image reconstruction unit 17.

[Step S5] The image reconstruction unit 17 uses the correction data sent from the correction data reading unit 21 to change the focal position of the X-ray tube 2 due to the rotation of the X-ray tube 2 and the inclination of the gantry 6. The image reconstruction is performed by performing the image signal processing while eliminating the fluctuation of the X-ray detection data caused.

[Step S6] Following the image reconstruction, the CT image signal of the section designated by the operator is stored in the CT image memory, and the CT image of the designated section is displayed on the screen of the display monitor 19 without artifacts. It is projected on.

[Step S7] If all the necessary CT images of the cross section have been obtained, the X-ray photography has been completed.

The present invention is not limited to the above embodiment, but can be modified as follows. (1) In the embodiment, a solid-state X-ray detector is used as the multi-channel X-ray detector.
The X-ray CT apparatus may use an ionization chamber type multi-channel X-ray detector. For example, an ion chamber type multi-channel X-ray detector 22 shown in FIG. 9 can be used. In the X-ray detector 22, for example, a partition plate 24 having both a channel collimator function and a voltage applying electrode function is installed at an inner channel boundary position in an ionization chamber 23 in which Xe gas is sealed, and an X-ray detector is provided between the partition plates. This is a structure in which the data extracting electrode 24 is provided.

In the X-ray detector 22, as shown in FIG. 10, as the focus position of the X-ray tube 2 changes, the fan beam FB changes from the state shown by the dashed line to the state shown by the dashed line. A shadow of the fan beam FB (a single hatched area in FIG. 10) is formed near the partition plate 24. However, in the case of the ionization chamber type X-ray detector 22, incident X-rays are almost absorbed and detected near the entrance (the hatched area in FIG. 10), so that the shadow of the fan beam FB is thin. ,
The error fluctuation of the X-ray detection data is smaller than that of the solid-state X-ray detector 22.

(2) In the embodiment, the X-ray caused by the change of the focal position of the X-ray tube is determined in accordance with the relationship between the variation of the X-ray detection data and the rotation angle of the X-ray tube and the tilt angle of the gantry.
The configuration is such that a correction operation for eliminating the fluctuation of the line detection data is performed. In the case of the X-ray CT apparatus of the present invention, if the error fluctuation of the X-ray detection data due to the change in the tilt angle of the gantry can be ignored, only the relationship between the fluctuation amount of the X-ray detection data and the rotation angle of the X-ray tube is obtained. A configuration may be employed in which a correction operation is performed to eliminate a change in X-ray detection data caused by a change in the focal position of the X-ray tube. In this case, the correction data memory stores the relationship (rotational variation characteristic) between the variation of the X-ray detection data and the rotation angle of the X-ray tube.

In the case of the X-ray CT apparatus of the present invention,
If the error fluctuation of the X-ray detection data due to the change in the rotation angle of the X-ray tube is negligible, it is caused by the change of the focal position of the X-ray tube according to only the relationship between the fluctuation amount of the X-ray detection data and the inclination angle of the gantry. A configuration may be employed in which a correction operation for eliminating fluctuations in X-ray detection data is performed. In this case, the correction data memory stores the relationship between the variation amount of the X-ray detection data and the tilt angle of the gantry (tilt variation characteristic).

[0042]

As described above in detail, according to the X-ray CT apparatus of the first aspect, the error fluctuation included in the X-ray detection data due to the rotation of the X-ray tube is stored in advance. Since the correction is performed by the correction operation performed according to the relationship between the variation amount of the X-ray detection data and the rotation angle of the X-ray tube, it is possible to prevent artifacts from appearing in the finally obtained CT image.

According to the X-ray CT apparatus of the second aspect,
Since the error fluctuation included in the X-ray detection data due to the inclination of the gantry is eliminated by the correction operation performed in accordance with the relationship between the fluctuation amount of the X-ray detection data stored in advance and the inclination angle of the gantry, the final variation is obtained. Artifacts can be prevented from appearing in the CT image obtained in step (1).

According to the X-ray CT apparatus of the third aspect,
Error fluctuations included in the X-ray detection data due to both the rotation of the X-ray tube and the inclination of the gantry are stored in the previously stored X-ray detection data.
The correction is performed according to the relationship between the amount of fluctuation of the line detection data and the rotation angle of the X-ray tube and the inclination angle of the gantry, so that artifacts can be prevented from appearing in the finally obtained CT image. .

Further, in the case of the X-ray CT apparatus according to the fourth aspect, the multi-channel type X-ray detector is a solid-element type multi-channel type X-ray detector, and the X-ray detection data has a remarkable error fluctuation. However, this remarkable error fluctuation is removed by the correction operation, and no artifact appears in the finally obtained CT image. Therefore, it is possible to effectively utilize a solid-state X-ray detector excellent in manufacturing suitability and the like. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an overall configuration of an X-ray CT apparatus according to an embodiment.

FIG. 2 is a schematic diagram showing a main part of a solid-state multi-channel X-ray detector of the apparatus of the embodiment.

FIG. 3 is a front view showing a tilting state of a gantry of the embodiment device.

FIG. 4 is a schematic diagram showing a force acting on the X-ray tube when the X-ray tube of the embodiment apparatus rotates.

FIG. 5 is a schematic diagram showing a force acting on the X-ray tube when the gantry of the embodiment apparatus is tilted.

FIG. 6 is a schematic diagram showing a change state of a focal position of an X-ray tube in the example apparatus.

FIG. 7 is a graph schematically illustrating an example of correction data stored in a correction data memory of the embodiment device.

FIG. 8 is a flowchart illustrating a flow of an X-ray imaging operation performed by the apparatus according to the embodiment.

FIG. 9 is a partial cross-sectional view showing a main configuration of an ionization chamber type multi-channel X-ray detector.

FIG. 10 is a schematic diagram showing an X-ray detection situation of an ionization chamber type multi-channel X-ray detector.

FIG. 11 is a schematic diagram showing a configuration around an imaging table of a conventional X-ray CT apparatus.

FIG. 12 shows a solid-state multi-channel X-ray detector X.
It is a schematic diagram which shows a line detection situation.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Top plate 2 ... X-ray tube 3 ... Solid-element type multi-channel type X-ray detector 4 ... Channel collimator 4a ... Partition plate 5 ... Solid state detection element 6 ... Gantry 8 ... Imaging system rotation drive unit 11 ... Gantry tilt drive 17 image reconstructing unit 20 correction data memory 21 correction data reading unit 22 ionization chamber type multi-channel X-ray detector FB fan beam (fan-shaped X-ray beam) M subject Z Body axis

Claims (4)

[Claims] An X-ray tube for irradiating a subject on a top plate with a fan-shaped X-ray beam, and a large number of X-ray detection elements for detecting transmitted X-rays of a portion determined by a channel collimator, respectively. A multi-channel X-ray detector arranged along the spread of the X-ray beam, and a body of the subject while maintaining a state in which the X-ray tube and the X-ray detector face each other with the subject sandwiched between the top plate A gantry equipped so as to be rotatable about an axis, and an X-ray detector as the X-ray tube emits a fan-shaped X-ray beam while rotating around the body axis of the subject. An X-ray CT apparatus comprising image reconstruction means for performing image signal processing on the X-ray detection data output from to perform image reconstruction for a CT image,
The relationship between the amount of change in X-ray detection data and the rotation angle of the X-ray tube caused by a change in the focal position of the X-ray tube with respect to the X-ray detector when the X-ray tube rotates around the body axis of the subject is stored in advance. Rotation fluctuation characteristic storage means,
The image reconstructing means is caused by a change in the focal position of the X-ray tube due to the rotation of the X-ray tube according to the relationship between the amount of fluctuation of the X-ray detection data stored in the rotation fluctuation characteristic storage means and the rotation angle of the X-ray tube. An X-ray CT apparatus configured to perform a correction operation for eliminating fluctuations in X-ray detection data by image signal processing. 2. An X-ray tube for irradiating a subject on a top plate with a fan-shaped X-ray beam, and a large number of X-ray detecting elements for detecting transmitted X-rays corresponding to a portion determined by a channel collimator. A multi-channel X-ray detector arranged along the spread of the X-ray beam, and a body of the subject while maintaining a state in which the X-ray tube and the X-ray detector face each other with the subject sandwiched between the top plate A gantry equipped so as to be rotatable about an axis, and an X-ray detector as the X-ray tube emits a fan-shaped X-ray beam while rotating around the body axis of the subject. An X-ray CT apparatus comprising image reconstruction means for performing image signal processing on the X-ray detection data output from to perform image reconstruction for a CT image,
Gantry tilting means for tilting the gantry so that a fan-shaped X-ray beam from the X-ray tube is irradiated obliquely to the subject, and the focal position of the X-ray tube with respect to the X-ray detector changes when the gantry is tilted And a tilt variation characteristic storage unit for storing in advance the relationship between the amount of variation of the X-ray detection data and the tilt angle of the gantry caused by the X-ray detection data.
In accordance with the relationship between the amount of change in the line detection data and the tilt angle of the gantry, a correction operation for eliminating the change in the X-ray detection data caused by the change in the focal position of the X-ray tube due to the tilt of the gantry is performed by image signal processing. X characterized by being
Line CT device. 3. An X-ray tube for irradiating a subject on a top plate with a fan-shaped X-ray beam and a large number of X-ray detecting elements for detecting transmitted X-rays corresponding to the respective areas determined by a channel collimator. A multi-channel X-ray detector arranged along the spread of the X-ray beam, and a body of the subject while maintaining a state in which the X-ray tube and the X-ray detector face each other with the subject sandwiched between the top plate A gantry equipped so as to be rotatable about an axis, and an X-ray detector as the X-ray tube emits a fan-shaped X-ray beam while rotating around the body axis of the subject. An X-ray CT apparatus comprising image reconstruction means for performing image signal processing on the X-ray detection data output from to perform image reconstruction for a CT image,
Gantry tilting means for tilting the gantry so that a fan-shaped X-ray beam from the X-ray tube is irradiated obliquely on the subject, and when the X-ray tube rotates around the body axis of the subject or tilts the gantry A rotation inclination variation characteristic storage unit for storing in advance a relationship between a variation amount of X-ray detection data caused by a change in a focal position of the X-ray tube with respect to the X-ray detector and a rotation angle of the X-ray tube and an inclination angle of the gantry. The image reconstructing means is provided for rotating the X-ray tube in accordance with the relationship between the amount of variation of the X-ray detection data stored in the rotation inclination variation characteristic storage means and the rotation angle of the X-ray tube and the gantry inclination angle. And a correction operation for eliminating a change in X-ray detection data caused by a change in a focal position of the X-ray tube due to a tilt of the gantry is performed by image signal processing. T devices. 4. The X-ray CT apparatus according to claim 1, wherein in the multi-channel X-ray detector, the X-ray detection element is a solid state detection element, and the channel collimator is a solid state detection element. An X-ray CT apparatus which is a solid-element type multi-channel X-ray detector comprising an X-ray partition plate disposed at each channel boundary position on the X-ray incidence side of (1).