JP4795584B2 - X-ray CT system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray computed tomography (X-ray CT) apparatus, and more particularly to an X-ray CT apparatus using an X-ray irradiation apparatus that irradiates a fan-shaped X-ray beam (beam) from a focal point having a variable size.
[0002]
[Prior art]
In the X-ray CT apparatus, transmission X-ray data (data) of a plurality of views (view) is acquired for an object to be imaged by an X-ray irradiation / detection apparatus, and a tomographic image of the object is acquired by an image generation apparatus based on the transmission X-ray data. Generate (reconfigure).
[0003]
The X-ray irradiation apparatus shapes a cone-shaped X-ray beam emitted from the focal point of an X-ray tube into a fan-shaped X-ray beam using a collimator and irradiates the imaging space.
[0004]
The X-ray detection apparatus is a multi-channel X-ray detection system in which a large number of detection elements are arranged in an array along the fan-shaped spread of the X-ray beam. Detect with instrument. By rotating (scanning) such an X-ray irradiation / detection device around an object, transmission X-ray data of a plurality of views is acquired.
[0005]
The slice thickness of the tomographic image is determined by the thickness of the X-ray beam. The thickness of the X-ray beam is adjusted by a collimator. Therefore, the slice thickness is adjusted by the collimator. The size of the focal point of X-rays is adjusted according to the slice thickness. When the slice thickness is reduced, the focal point is reduced, and when the slice thickness is increased, the focal point is increased.
[0006]
[Problems to be solved by the invention]
When the focal point is reduced, the concentration of the electron beam at the anode of the X-ray tube increases and the temperature rise of the X-ray tube increases. For this reason, when the focus is reduced, it is necessary to limit the time integration of tube current, that is, so-called mAs, or to stop scanning for cooling the X-ray tube.
[0007]
Therefore, an object of the present invention is to realize an X-ray CT apparatus with few mAs and cooling restrictions even when the X-ray focal point is reduced.
[0008]
[Means for Solving the Problems]
(1) In one aspect of the invention for solving the above-described problem, an X-ray irradiation apparatus that irradiates a fan-shaped X-ray beam from a focal point having a variable size, and a plurality of X-ray detection elements in the fan shape. An X-ray irradiation / detection device having an X-ray detection device arranged in the direction of the X-ray beam and facing the X-ray irradiation device across the object to be imaged is rotated around the object to be imaged Transmission X-ray data acquisition means for acquiring transmission X-ray data of a plurality of views for the object, tomogram generation means for generating a tomogram of the object based on the acquired transmission X-ray data, and the tomogram generation means Focus adjusting means for adjusting the size of the focus of the X-ray beam according to the tomographic image generation conditions of the X-ray CT apparatus.
[0009]
In the invention according to the aspect described in (1), since the focus size of the X-ray beam is adjusted according to the tomographic image generation conditions, the focus size is adjusted according to the slice thickness as in the prior art. In comparison, the occurrence frequency of the situation of reducing the focus is reduced, and the restrictions on mAs and cooling are eased.
[0010]
It is preferable that the tomographic image generation condition relates to spatial frequency enhancement in terms of reducing the frequency of occurrence of a situation where the focal point is reduced.
When the spatial frequency emphasis is high-frequency emphasis, it is preferable that the focus adjusting means reduce the focus in terms of reducing the occurrence frequency of the situation of reducing the focus.
[0011]
It is preferable that the focus adjustment means increase the focus when the spatial frequency emphasis is low-frequency emphasis in terms of reducing the occurrence frequency of the situation of reducing the focus.
It is preferable that the focus adjusting means reduce the focus when the spatial frequency enhancement is high-frequency enhancement and increase the focus when the spatial frequency enhancement is low-frequency enhancement, from the viewpoint of reducing the occurrence frequency of the situation of reducing the focus.
[0012]
(2) Another aspect of the invention for solving the above problems is an X-ray irradiation apparatus that irradiates a fan-shaped X-ray beam having a variable thickness from a focal point having a variable size, and a plurality of X-ray detections. An X-ray irradiating / detecting device having an X-ray irradiating device opposite to the X-ray irradiating device with elements arranged in the direction of the fan-shaped X-ray beam spread around the photographic subject A transmission X-ray data acquisition means for acquiring transmission X-ray data of a plurality of views with respect to the object, a tomogram generation means for generating a tomogram of the object based on the acquired transmission X-ray data, And an X-ray CT apparatus comprising: a focus adjusting unit that adjusts a size of a focal point of the X-ray beam according to a thickness of the X-ray beam and a tomographic image generation condition of the tomographic image generating unit. .
[0013]
In the invention according to the aspect described in (2), since the size of the focus of the X-ray beam is adjusted according to the thickness of the X-ray beam and the tomographic image generation conditions, the size of the focus is determined according to the slice thickness as in the prior art. Compared with adjusting the height, the frequency of occurrence of reducing the focal point decreases, and the restrictions on mAs and cooling are relaxed.
[0014]
It is preferable that the tomographic image generation condition relates to spatial frequency enhancement in terms of reducing the frequency of occurrence of a situation where the focal point is reduced.
It is preferable that the focus adjusting means reduce the focus when the thickness of the X-ray beam is thin and the spatial frequency emphasis is high-frequency emphasis in terms of reducing the occurrence frequency of the situation of reducing the focus.
[0015]
It is preferable that the focus adjusting means reduce the focus when the thickness of the X-ray beam is thick and the spatial frequency emphasis is high frequency emphasis from the viewpoint of reducing the occurrence frequency of the situation of reducing the focus.
[0016]
It is preferable that the focus adjusting means increase the focus when the thickness of the X-ray beam is thin and the spatial frequency emphasis is low-frequency emphasis from the viewpoint of reducing the occurrence frequency of the situation of reducing the focus.
[0017]
It is preferable that the focus adjusting means increase the focus when the thickness of the X-ray beam is thick and the spatial frequency emphasis is low-frequency emphasis from the viewpoint of reducing the occurrence frequency of the situation of reducing the focus.
[0018]
When the spatial frequency enhancement is high-frequency enhancement, the focus adjustment means reduces the focus when the spatial frequency enhancement is high-frequency enhancement and increases the focus when the spatial frequency enhancement is low-frequency enhancement. This is preferable in that the frequency decreases.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the embodiment. FIG. 1 shows a block diagram of an X-ray CT apparatus. This apparatus is an example of an embodiment of the present invention. An example of an embodiment relating to the apparatus of the present invention is shown by the configuration of the apparatus.
[0020]
As shown in FIG. 1, the apparatus includes a scanning gantry 2, an imaging table 4, and an operation console 6.
The scanning gantry 2 is an example of an embodiment of transmission X-ray data acquisition means in the present invention. The scanning gantry 2 has an X-ray tube 20. X-rays (not shown) radiated from the X-ray tube 20 are shaped by the collimator 22 into, for example, a fan-shaped X-ray beam, that is, a fan beam, and irradiated to the X-ray detector 24. The portion composed of the X-ray tube 20 and the collimator 22 is an example of an embodiment of the X-ray irradiation apparatus in the present invention.
[0021]
The X-ray detector 24 has a plurality of detection elements arranged in an array in the direction of expansion of the fan-shaped X-ray beam. The X-ray detector 24 is an example of an embodiment of an X-ray detection apparatus according to the present invention. The configuration of the X-ray detector 24 will be described later.
[0022]
The X-ray tube 20, the collimator 22, and the X-ray detector 24 constitute an X-ray irradiation / detection device. The X-ray irradiation / detection device is an example of an embodiment of the X-ray irradiation / detection device according to the present invention. The X-ray irradiation / detection apparatus will be described later.
[0023]
A data collection unit 26 is connected to the X-ray detector 24. The data collection unit 26 collects detection signals of individual detection elements of the X-ray detector 24 as digital data.
[0024]
X-ray irradiation from the X-ray tube 20 is controlled by an X-ray controller 28. The size of the X-ray focal point in the X-ray tube 20 is also adjusted by the X-ray controller 28. The size of the focal point is adjusted in two steps, for example, by selecting a cathode. The connection relationship between the X-ray tube 20 and the X-ray controller 28 is not shown.
[0025]
The collimator 22 is controlled by a collimator controller 30. The collimator controller 30 adjusts the thickness of the X-ray beam, that is, the slice thickness, through the collimator 22. The connection relationship between the collimator 22 and the collimator controller 30 is not shown.
[0026]
The components from the X-ray tube 20 to the collimator controller 30 described above are mounted on the rotating unit 34 of the scanning gantry 2. The rotation of the rotating unit 34 is controlled by the rotation controller 36. The connection relationship between the rotating unit 34 and the rotation controller 36 is not shown.
[0027]
The imaging table 4 carries an imaging target (not shown) in and out of the X-ray irradiation space of the scanning gantry 2. The relationship between the object and the X-ray irradiation space will be described later.
[0028]
The operation console 6 has a data processing device 60. The data processing device 60 is configured by, for example, a computer. A control interface (interface) 62 is connected to the data processing device 60. The control gantry 2 and the imaging table 4 are connected to the control interface 62. The data processing device 60 controls the scanning gantry 2 and the imaging table 4 through the control interface 62.
[0029]
The data acquisition unit 26, the X-ray controller 28, the collimator controller 30 and the rotation controller 36 in the scanning gantry 2 are controlled through the control interface 62. Note that illustration of individual connections between these units and the control interface 62 is omitted.
[0030]
A data collection buffer 64 is also connected to the data processing device 60. A data collection unit 26 of the scanning gantry 2 is connected to the data collection buffer 64. Data collected by the data collection unit 26 is input to the data processing device 60 through the data collection buffer 64.
[0031]
The data processing device 60 performs image reconstruction using transmission X-ray data of a plurality of views collected through the data collection buffer 64. For the image reconstruction, for example, a filtered back projection method or the like is used. The data processing device 60 is an example of an embodiment of tomographic image generation means in the present invention.
[0032]
A storage device 66 is also connected to the data processing device 60. The storage device 66 stores various data, programs, and the like. When the data processing device 60 executes the program stored in the storage device 66, a reconstructed image is performed, and various data processing described later is performed. The reconstructed image is stored in the storage device 66.
[0033]
A display device 68 and an operation device 70 are also connected to the data processing device 60. The display device 68 displays the reconstructed image and other information output from the data processing device 60. The operation device 70 is operated by a user and inputs various instructions and information to the data processing device 60. The user operates the present apparatus interactively using the display device 68 and the operation device 70.
[0034]
FIG. 2 shows a schematic configuration of the X-ray detector 24. As shown in the figure, the X-ray detector 24 is a multi-channel X-ray detector in which a plurality of detection elements 24 (i) are arranged in an array.
[0035]
The plurality of detection elements 24 (i) as a whole form an X-ray incident surface curved in a cylindrical concave shape. i is a channel number, for example, i = 1 to 1000. The detection element 24 (i) is configured by, for example, a combination of a scintillator and a photodiode. However, the present invention is not limited to this. For example, a semiconductor detection element using cadmium tellurium (CdTe) or an ionization chamber type detection element using Xe gas (gas) may be used.
[0036]
FIG. 3 shows the interrelationship among the X-ray tube 20, the collimator 22, and the X-ray detector 24 in the X-ray irradiation / detection apparatus. 3A is a diagram showing a state seen from the front of the scanning gantry 2, and FIG. 3B is a diagram showing a state seen from the side.
[0037]
As shown in the figure, the X-rays radiated from the X-ray tube 20 are shaped by the collimator 22 into a fan-shaped X-ray beam 400 and irradiated to the X-ray detector 24. 3A shows the spread of the fan-shaped X-ray beam 400, and FIG. 3B shows the thickness of the X-ray beam 400.
[0038]
The body axis is intersected with the fan surface of the X-ray beam 400, and the object 8 placed on the imaging table 4 is carried into the X-ray irradiation space, for example, as shown in FIG. The scanning gantry 2 has a cylindrical structure including an X-ray irradiation / detection device inside.
[0039]
The X-ray irradiation space is formed in the inner space of the cylindrical structure of the scanning gantry 2. An image of the object 8 sliced by the X-ray beam 400 is projected onto the X-ray detector 24. X-rays transmitted through the object 8 are detected by the X-ray detector 24. As shown in FIG. 5, the thickness of the X-ray beam 400 irradiated to the object 8, that is, the slice thickness th is adjusted by the opening degree of the aperture of the collimator 22.
[0040]
The X-ray irradiation / detection device including the X-ray tube 20, the collimator 22, and the X-ray detector 24 rotates (scans) around the body axis of the object 8 while maintaining their mutual relationship.
[0041]
Projection data of a plurality of views (for example, about 1000) per scan rotation is collected. Projection data is collected by the system of X-ray detector 24 -data collection unit 26 -data collection buffer 64.
[0042]
Based on, for example, 1000 views of projection data collected in the data collection buffer 64, the data processing device 60 generates a tomographic image, that is, image reconstruction. Image reconstruction is performed by processing, for example, 1000 views of projection data obtained by one rotation scan, for example, by a filtered back projection method.
[0043]
Spatial frequency enhancement processing for a tomographic image differs depending on the part to be imaged. For example, high-frequency enhancement is performed on a tomographic image or a lung field tomographic image related to a head bone structure including an ossicle. On the other hand, low-frequency emphasis is performed on the head angiographic image and the abdominal tomographic image. High frequency enhancement is performed by image reconstruction using a high frequency enhancement function, and low frequency enhancement is performed by image reconstruction using a low frequency enhancement function.
[0044]
FIG. 6 shows an example of the gain profile of the high frequency enhancement function and the low frequency enhancement function. In the figure, the horizontal axis is the spatial frequency, and fN is the Nyquist frequency. As shown in the figure, the high frequency enhancement function has a gain up to the high frequency within a range not exceeding the Nyquist frequency. On the other hand, the low frequency enhancement function has a gain only in the low frequency.
[0045]
High-frequency enhancement is performed to obtain a tomographic image with high spatial resolution. When obtaining a tomographic image with high spatial resolution, the slice thickness is reduced and the X-ray focal point is reduced.
[0046]
The low-frequency emphasis is performed in order to obtain a tomographic image with good SNR (signal-to-noise ratio) by reducing noise. Since the spatial resolution is reduced by low-frequency enhancement, the effect does not appear even when the focus of X-rays is reduced when a low-frequency enhanced image is taken. Therefore, when taking a low-frequency emphasized image, the X-ray focus may be increased regardless of the slice thickness.
[0047]
Based on this concept, photographing with this apparatus is performed as follows. FIG. 7 shows a flow chart of the operation of this apparatus. As shown in the figure, in step 702, shooting conditions are set. The photographing condition is set by the user through the operation device 70.
[0048]
With this imaging condition setting, the tube voltage, tube current, slice thickness, imaging region, etc. of the X-ray tube are set. In addition, the spatial frequency enhancement function is automatically set with the setting of the imaging region. Alternatively, the user may select and set. As a result, for example, a high-frequency enhancement function is set when imaging the ossicular portion or lung field, and a low-frequency enhancement function is set when performing head angiography or abdominal tomography.
[0049]
Next, in step 704, it is determined whether the frequency enhancement function is high frequency enhancement. This determination is performed by the data processing device 60. If the result of determination is Yes, a small focus is set in step 706, and if no, a large focus is set in step 708.
[0050]
Such determination and focus setting based on the determination are performed by the data processing device 60. The data processing device 60 is an example of an embodiment of the focus adjusting means in the present invention.
[0051]
Next, a scan is performed at step 710. Scanning is performed under the imaging conditions and X-ray focal point set as described above. Due to the focus setting as described above, a small focus is used only when a high-frequency emphasized tomographic image is taken.
[0052]
For this reason, as compared with the conventional case where a small focus is used regardless of frequency enhancement when the slice thickness is thin, the requirement for high frequency enhancement is further added, thereby reducing the frequency of using the small focus. In other words, even when the slice thickness is thin, a large focal point is used when capturing a low-frequency emphasized tomographic image, so the frequency of occurrence of a situation where a small focal point is used correspondingly decreases.
[0053]
The reduced frequency of using small focal points reduces the frequency of competing with the mAs limit and also reduces the frequency of stopping scans due to x-ray tube cooling.
[0054]
Based on the transmitted X-ray data collected by such a scan, image reconstruction is performed in step 712, and then the reconstructed image is displayed and stored in step 714.
[0055]
In addition, the slice thickness is also added to the determination condition in step 704, and as shown in FIG. 8, a small focus is obtained when high-frequency emphasis is applied with a thin slice thickness, a large focus is applied when thin slice thickness is applied with low-frequency emphasis, and a thick slice thickness is obtained. A small focus may be used for high frequency emphasis, and a large focus may be used for low frequency emphasis with a thick slice thickness.
[0056]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to realize an X-ray CT apparatus with less restrictions on mAs and cooling even when the X-ray focal point is reduced.
[Brief description of the drawings]
FIG. 1 is a block diagram of an exemplary apparatus according to an embodiment of the present invention.
FIG. 2 is a schematic diagram of an X-ray detector.
FIG. 3 is a schematic diagram of an X-ray irradiation / detection apparatus.
FIG. 4 is a schematic diagram of an X-ray irradiation / detection apparatus.
FIG. 5 is a schematic diagram of an X-ray irradiation / detection apparatus.
FIG. 6 is a diagram of a gain profile of a spatial frequency enhancement function.
FIG. 7 is a flowchart of the operation of the apparatus according to the exemplary embodiment of the present invention.
FIG. 8 is a diagram illustrating an example of focus setting.
[Explanation of symbols]
2 Scanning gantry 20 X-ray tube 22 Collimator 24 X-ray detector 26 Data acquisition unit 28 X-ray controller 30 Collimator controller 34 Rotation unit 36 Rotation controller 4 Imaging table 6 Operation console 60 Data processing device 62 Control interface 64 Data acquisition buffer 66 Storage Device 68 Display device 70 Operating device 8 Target 400 X-ray beam

Claims (11)

An X-ray irradiation apparatus that irradiates a fan-shaped X-ray beam from a focal point having a variable size, and a plurality of X-ray detection elements arranged in the direction of expansion of the fan-shaped X-ray beam so as to sandwich an object to be imaged Transmission X-ray data acquisition means for acquiring transmission X-ray data of a plurality of views for the object by rotating an X-ray irradiation / detection apparatus having an X-ray detection apparatus facing the X-ray irradiation apparatus around the object to be imaged When,
Tomographic image reconstruction means for reconstructing the tomographic image of the object based on the acquired transmitted X-ray data;
A focus adjustment means for adjusting the magnitude of the focal point of the X-ray beam in accordance with the tomographic image reconstruction conditions of the tomographic image reconstruction means,
An X-ray CT apparatus comprising: The tomographic reconstruction conditions are related to spatial frequency enhancement.
The X-ray CT apparatus according to claim 1. The focus adjusting means reduces the focus when the spatial frequency emphasis is high frequency emphasis;
The X-ray CT apparatus according to claim 2. The X-ray CT apparatus according to claim 3, further comprising setting means for setting the spatial frequency emphasis to high frequency emphasis when photographing an ossicle or a lung field. The focus adjusting means increases the focus when the spatial frequency emphasis is low-frequency emphasis;
The X-ray CT apparatus according to claim 2. 6. The X-ray CT apparatus according to claim 5, further comprising setting means for setting the spatial frequency enhancement to low-frequency enhancement when performing head angiography or abdominal tomography. The focus adjusting means reduces the focus when the spatial frequency emphasis is high frequency emphasis and increases the focus when the low frequency emphasis is;
The X-ray CT apparatus according to claim 2. An X-ray irradiation apparatus that irradiates a fan-shaped X-ray beam having a variable thickness from a focal point having a variable size, and a plurality of X-ray detection elements arranged in the direction of expansion of the fan-shaped X-ray beam. Transmission X-ray X-ray irradiating / detecting device having an X-ray irradiating device facing the X-ray irradiating device across the object is rotated around the object to be imaged to obtain transmission X-ray data of a plurality of views for the object Line data acquisition means;
Tomographic image reconstruction means for reconstructing the tomographic image of the object based on the acquired transmitted X-ray data;
X-ray CT which is characterized by comprising: a focus adjustment means for adjusting the magnitude of the focal point of the X-ray beam in accordance with the tomographic image reconstruction conditions of the thickness and the tomographic image reconstruction means of the X-ray beam apparatus. The tomographic reconstruction conditions are related to spatial frequency enhancement.
The X-ray CT apparatus according to claim 8 . The focus adjusting means reduces the focus when the thickness of the X-ray beam is thin and the spatial frequency enhancement is high-frequency enhancement, and focuses when the thickness of the X-ray beam is thick and the spatial frequency enhancement is high-frequency enhancement. The focus is increased when the thickness of the X-ray beam is thin and the spatial frequency enhancement is low-frequency enhancement, and the focus is increased when the thickness of the X-ray beam is thick and the spatial frequency enhancement is low-frequency enhancement. The X-ray CT apparatus according to claim 9, wherein: The focus adjusting means reduces the focus when the spatial frequency enhancement is high-frequency enhancement and increases the focus when low-frequency enhancement is performed regardless of the thickness of the X-ray beam.
The X-ray CT apparatus according to claim 9 .

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