JP3977624B2 - X-ray computed tomography system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray computed tomography apparatus.
[0002]
[Prior art]
As is well known, an X-ray computed tomography apparatus is an apparatus that realizes reconstruction of a cross-sectional attenuation coefficient distribution, that is, a tomographic image, based on projection data acquired by scanning a subject with X-rays from multiple directions. It is.
[0003]
In recent years, the development of semiconductor detectors that directly convert X-rays that have passed through a subject into charges has progressed, and along with the improvement in sensitivity, the reduction in X-ray dose, that is, the reduction in exposure dose has progressed.
[0004]
However, there is a limit to the reduction of the exposure dose. If the exposure dose is less than a certain amount, the density resolution is remarkably lowered, and it is impossible to secure an image quality that can withstand diagnosis.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to achieve both reduction of exposure dose and ensuring high image quality in an X-ray computed tomography apparatus.
[0006]
[Means for Solving the Problems]
In the first aspect of the present invention, the first X-ray tube is mounted on a substantially annular rotating mount and irradiates the subject with X-rays, and is disposed between the subject and the first X-ray tube. A first X-ray aperture device with variable opening, a first X-ray detector mounted at a position facing the first X-ray tube on the rotary mount, and the first X-ray tube on the rotary mount A second X-ray tube device mounted at a position deviated by a predetermined angle in the circumferential direction; a second X-ray diaphragm device with variable opening disposed between the subject and the second X-ray tube; and the rotation A second X-ray detector mounted on a gantry at a position facing the second X-ray tube device, and the second X-ray diaphragm device so that the X-ray is irradiated only to the region of interest of the subject . a control unit for setting narrower than the opening degree of the first 1X-ray beam limiting device the opening, of the channels of the first 2X-ray detector A narrow due to opening of the projection data of the channel that caused the data loss, and data interpolation unit for complementing each rotation angle projection data of the corresponding channel of the second 1X-ray detector, according to the first 2X-ray detector And a reconstruction unit configured to reconstruct image data based on projection data and the projection data by the first X-ray detector to be complemented .
According to a second aspect of the present invention, in the X-ray computed tomography apparatus equipped with a plurality of pairs of an X-ray tube apparatus and an X-ray detector, at least one pair of X-ray tubes among the plurality of pairs is provided. The X-ray spread angle is set to be narrower than that of the other pairs of X-ray tube devices so that the X-ray is irradiated only to the region of interest of the subject, and the X-ray spread angle is narrower. The image data is reconstructed based on the set projection data of at least one pair and the projection data of the other pair that complements the missing channel in the at least one pair.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the X-ray computed tomography apparatus according to the present invention will be described below with reference to the drawings. As a scanning method of the X-ray computed tomography apparatus, there are a rotation / rotation type in which an X-ray tube and an X-ray detector are rotated as one body, and a large number of detection elements arrayed in a ring shape. There are various types such as a stationary / rotate type in which only the X-ray tube rotates around the subject, and the present invention can be applied to any type, but here, the rotate / rotate type will be described as an example. . In addition, to reconstruct one tomographic image data, one set of projection data for about 360 ° around the periphery of the subject is required, and projection data for 180 ° + fan angle is also required for the half-scan method. It is said. Here, the former example will be described. The projection data is defined as integral data relating to the passing distance of the attenuation coefficient (or absorption coefficient) of the tissue or the like on the X-ray path.
[0009]
FIG. 1 shows the configuration of the main part of the X-ray computed tomography apparatus according to this embodiment. The X-ray computed tomography apparatus according to this embodiment includes a scan gantry 1, a computer apparatus 2, and a bed (not shown). The scan gantry 1 is a component for collecting projection data relating to a subject, and the projection data is captured by the computer device 2 and used for processing such as image reconstruction. The subject is inserted into the imaging region of the scan gantry 1 while being placed on the couch top.
[0010]
The computer apparatus 2 includes a central control unit 21, a data complementing unit 23, an image reconstruction unit 24 and an image display unit 25 connected to the central control unit 21 via a data / control bus 22.
[0011]
The scan gantry 1 is a multi-tube sphere type, that is, a plurality of pairs of X-ray tube devices and X-ray detectors are mounted on an annular rotary mount. Here, a two-tube type will be described. In the first pair 11, a first X-ray tube device 110 and a multi-channel first X-ray detector 113 facing the first X-ray tube device 110 are mounted on a rotating mount. The second pair 12 includes a second X-ray tube device 120 and a multi-channel second X-ray detector 123 facing the second X-ray tube device 120, the central axis of which is a predetermined angle between the central axis of the first pair 11 and the rotation axis RA (here Are assumed to be 90 °) and are mounted on the rotating base in a positional relationship where the same cross section (slice) as the first pair 11 is scanned.
[0012]
The first X-ray tube apparatus 110 is disposed between the first X-ray tube 111 and the first X-ray tube 111 and the subject, specifically, immediately before the X-ray emission window of the first X-ray tube 111. And peripheral elements such as the first X-ray diaphragm device 120 attached to the lens. The first X-ray diaphragm device 120 is a device for limiting the spread angle (also referred to as fan angle or viewing angle) of X-rays radiated from the first X-ray tube 111, and includes a plurality of movable shielding plates, And a drive unit for individually moving them. By controlling the position of each of the plurality of movable shielding plates, the opening degree of the opening and the center position of the opening can be arbitrarily adjusted.
[0013]
Similarly, the second X-ray tube device 120 includes a second X-ray tube 121 and peripheral elements such as the second X-ray diaphragm device 122. Similarly to the first X-ray diaphragm device 112, the second X-ray diaphragm device 122 is a device for limiting the spread angle (fan angle) of X-rays radiated from the second X-ray tube 121. A movable shielding plate and a drive unit that individually moves the movable shielding plate are provided. By controlling the position of each of the plurality of movable shielding plates, the opening degree and the center position of the opening can be arbitrarily adjusted. It is like that.
[0014]
The X-ray control unit 13 can apply and supply a tube voltage and a filament current (the tube current is controlled by the filament current) to the first and second X-ray tubes 111 and 121 individually. Two high voltage generators corresponding to the first X-ray tube 111 and the second X-ray tube 121 are provided. The tube voltage and the filament current applied to the first and second X-ray tubes 111 and 121 from these two high voltage generators are under the control of the scan control unit 14. Along with the control on the X-ray control unit 13, the scan control unit 14 opens the opening and the center position of each of the first and second X-ray diaphragm devices 112 and 122, and further rotates the rotating base and slides the top plate. Responsible for all operation control related to equal scan.
[0015]
Outputs of the first and second X-ray detectors 113 and 123 are supplied to the computer apparatus 2 as projection data via data collection units 114 and 124, a slip ring and a preprocessing unit (not shown) that enable continuous rotation, respectively. The The data complementing unit 23 of the computer apparatus 2 supplements the missing portion of the projection data of the second X-ray detector 123 with the projection data of the first X-ray detector 113, as will be described in detail later. Based on the complemented projection data, tomographic image data is reconstructed by the reconstruction unit 24 and displayed on the image display unit 25.
[0016]
FIG. 2 shows the opening degree of the first X-ray diaphragm device 112 and the opening degree of the second X-ray diaphragm device 122 adjusted by the scan control unit 14. The opening degree of the second X-ray diaphragm device 122 is set larger than the opening degree of the first X-ray diaphragm device 112. The opening A1 of the first X-ray diaphragm 112 is set to be relatively wide so that the X-ray from the first X-ray tube 111 irradiates the entire cross section of the subject, while the opening of the second X-ray diaphragm 122 is A2 is set to be relatively narrow such that the X-ray from the second X-ray tube 121 irradiates only the region of interest (region of interest) within the subject cross section and does not irradiate the portion other than the region of interest. . Actually, the respective openings A1 and A2 may be set to standard values according to parameters such as the physique of the subject and the type of region of interest, or tomographic images obtained by prescanning with low exposure. To the subject individual value.
[0017]
Thus, scanning is performed at different opening degrees. During scanning, the dose of X-rays exposed from the first X-ray tube 111 is set lower than the dose of X-rays exposed from the second X-ray tube 121. Specifically, the tube voltage TV1 applied to the first X-ray tube 111 and the tube voltage TV1 applied to the second X-ray tube 121 are set to be equivalent or substantially equivalent, while the first X-ray tube The filament current 111 (tube current: TC1) is set lower than the filament current (tube current: TC2) of the second X-ray tube 121. For example, the tube current of the second X-ray tube 121 is set to 250 mA, and the tube current of the first X-ray tube 111 is set to 50 mA. In this way, by making the tube voltage the same and making the tube current different, the dose is different, but the radiation quality can be made the same.
[0018]
Scanning is performed under such settings, and projection data detected by the first and second X-ray detectors 113 and 123 is supplied to the data complementing unit 23 of the computer apparatus 2. Since the X-rays of the second pair 12 do not cover the entire cross section of the subject, the tomographic image cannot be substantially reconstructed only with the projection data detected by the second X-ray detector 123. That is, data loss is caused in some of the channels of the second X-ray detector 123 due to a narrow opening. The data complementing unit 23 performs a process of complementing the missing channel data with the projection data of the corresponding channel of the first X-ray detector 113.
[0019]
FIG. 3 shows the trajectories of the first and second X-ray tubes 111 and 121, respectively. Since the second X-ray tube 121 is disposed at a position delayed by 90 ° with respect to the first X-ray tube 111, a trajectory delayed from the first X-ray tube 111 by the angle is drawn. Here, attention is paid to the rotation angle α °. FIG. 4 shows a projection data distribution detected by the second X-ray detector 123 when the second X-ray tube 121 is at the rotation angle α °. It can be seen that the data of the channel of the part not irradiated with X-rays is missing. The projection data of the channel causing the data loss is complemented by the projection data of the same channel detected by the first X-ray detector 113 when the first X-ray tube 111 is at the same rotation angle α °.
[0020]
Actually, since the doses are different and the reference levels are different accordingly, weighted addition is performed in order to align the reference levels. When the rotation angle is θ, the channel is ch, the projection data detected by the first X-ray detector 113 is PD1, and the projection data detected by the second X-ray detector 123 is PD2, the projection output from the complementing unit 23 Data PD is represented as follows.
[0021]
PD (θ, ch) = a · PD1 (θ, ch) + b · PD2 (θ, ch)
a and b are weighting factors, and the relationship is such that a> b and the dose ratio of the first X-ray tube 111 to the second X-ray tube 121 is c, so that a = c · b is satisfied. Is set. For example, similarly to the tube voltage and tube current conditions described above, assuming that the dose ratio c is (50 mA / 250 mA) = 1/5, a = 5 and b = 1 are set.
[0022]
Actually, a = 5 and b = 0 in the channel where data is lost in the second pair 12, and a = 0 and b = 1 in the channel where data is not lost in the second pair 12. Typically, only the channel in which data loss has occurred is supplemented with the projection data collected in the first pair 11.
[0023]
By combining X-rays with a narrow fan angle but high dose, and X-rays with a wide fan angle but low dose, high resolution and high image quality are provided for the necessary part, that is, the region of interest. However, the exposure dose can be kept low as compared with the irradiation of the X-ray with a high dose to the region of interest and the entire other portion.
[0024]
In the actual imaging procedure, pre-scanning is performed using either one of the pairs 11 or 12 under the condition of irradiating the entire cross section of the subject with a low dose of X-rays, and the relatively low resolution acquired thereby. The region of interest is identified from the tomographic image, and the opening degree of the second X-ray diaphragm device 122 is set based on the size, and as shown in FIG. It is considered necessary to secure a state in which the region of interest is arranged. In this state, the opening position of the second X-ray diaphragm device 122 may be fixed during rotation.
[0025]
However, as shown in FIG. 6, it cannot be said that there is no case where it is impossible or difficult to secure the state in which the region of interest is arranged on the rotation axis RA. In this case, it is necessary to set a wide opening so that the region of interest is always captured within the X-ray field of view regardless of the rotation angle of the second X-ray tube 121 in one rotation. However, with this setting, the exposure dose cannot be minimized. Therefore, after setting the opening of the second X-ray diaphragm 122 to a narrow opening that is narrowed down to the size of the region of interest, the region of interest is always set regardless of the rotation angle of the second X-ray tube 121 in one rotation. It is effective to move the opening position of the second X-ray aperture device 122 in synchronization with the rotation angle so that the X-ray is captured in a narrow field of view of the X-ray. The relationship between the opening position of the second X-ray diaphragm 122 and the rotation angle is determined by the position of the region of interest on the tomographic image acquired by the pre-scan, the position of the rotation axis RA, and the focal point of the second X-ray tube 121. It can be obtained geometrically from the positional relationship of the three with the position. In accordance with the relationship between the opening position of the second X-ray diaphragm device 122 and the rotation angle, the scan controller 14 causes the second X-ray diaphragm device 122 to move the opening position of the second X-ray diaphragm device 122 in synchronization with the rotation angle. Can be controlled.
[0026]
The present invention is not limited to the embodiment described above, and can be implemented with various modifications.
[0027]
【The invention's effect】
According to the present invention, in the X-ray computed tomography apparatus, it is possible to achieve both reduction in exposure dose and ensuring high image quality.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a main part of an X-ray computed tomography apparatus according to an embodiment of the present invention.
2 is a view showing the opening degree of each of the first and second X-ray diaphragm devices of FIG. 1; FIG.
FIG. 3 is a diagram showing trajectories of the first and second X-ray tubes shown in FIG. 1;
4 is a diagram schematically showing projection data of each of the first and second pairs in FIG. 1. FIG.
FIG. 5 is a diagram illustrating a state in which the opening degree and the opening position of the rotating second X-ray diaphragm device do not change when a region of interest exists on the rotation axis in the present embodiment.
FIG. 6 is a diagram illustrating a state in which the opening degree and the opening position of the second X-ray diaphragm device change with rotation when a region of interest exists at a position shifted from the rotation axis in the present embodiment.
[Explanation of symbols]
1 ... Scan gantry,
11 ... 1st pair,
110 ... first X-ray tube device,
111 ... 1st X-ray tube,
112 ... 1st X-ray aperture apparatus,
113 ... 1st detector,
114 ... 1st data collection part,
12 ... The second pair,
120 ... the second X-ray tube device,
121 ... the second X-ray tube,
122 ... the second X-ray diaphragm device,
123 ... the second detector,
124 ... the second data collection unit,
13 ... X-ray control unit,
14: Scan control unit,
2 ... Computer device,
21 ... Central control unit,
22: Data / control bus,
23: Data interpolation unit,
24 ... Reconstruction unit,
25: Image display unit.

Claims (8)

A first X-ray tube mounted on a substantially annular rotary mount for irradiating a subject with X-rays;
A variable first opening X-ray diaphragm device disposed between the subject and the first X-ray tube;
A first X-ray detector mounted at a position facing the first X-ray tube on the rotating mount;
A second X-ray tube device mounted at a position deviated from the first X-ray tube on the rotating mount by a predetermined angle in the circumferential direction;
A second X-ray diaphragm with variable opening disposed between the subject and the second X-ray tube;
A second X-ray detector mounted at a position facing the second X-ray tube device on the rotating mount;
A control unit for setting the opening of the second X-ray diaphragm device to be narrower than the opening of the first X-ray diaphragm device so that the X-ray is irradiated only to the region of interest of the subject .
Data that complements projection data of a channel in which data is lost due to a narrow opening degree among channels of the second X-ray detector for each rotation angle with projection data of a corresponding channel of the first X-ray detector. Complement part,
An X-ray computed tomography comprising: a reconstruction unit that reconstructs image data based on projection data from the second X-ray detector and complemented projection data from the first X-ray detector apparatus.   The X-ray computed tomography apparatus according to claim 1, further comprising an X-ray controller configured to set an X-ray dose of the second X-ray tube higher than an X-ray dose of the first X-ray tube.   The X-ray computed tomography apparatus according to claim 1, further comprising an X-ray controller configured to set a tube current of the second X-ray tube higher than a tube current of the first X-ray tube.   4. The X-ray computed tomography apparatus according to claim 3, further comprising an X-ray control unit that sets tube voltages of the first and second X-ray tubes to be substantially equivalent.   2. The X-ray computed tomography apparatus according to claim 1, wherein the control unit changes the position of the opening of the second X-ray diaphragm device with rotation. 3. In an X-ray computed tomography apparatus equipped with a plurality of pairs of an X-ray tube apparatus and an X-ray detector,
At least one pair of X-ray tube devices among the plurality of pairs is more X-ray than other pairs of X-ray tube devices so that the X-rays irradiate only the region of interest of the subject. The spread angle is set to be narrow,
Image data is reconstructed based on projection data of at least one pair in which the spread angle of the X-ray is set narrow and projection data of the other pair that complements a channel that is missing in the at least one pair An X-ray computed tomography apparatus. 7. The X-ray computed tomography according to claim 6 , wherein the X-ray tube is set to have a higher tube current in the pair in which the X-ray spread angle is set narrower than that in the other pair. apparatus. 8. The X-ray computed tomography according to claim 7 , wherein the pair in which the X-ray spread angle is set narrow is set to be substantially equivalent to the other pair in the tube voltage of the X-ray tube. Shooting device.

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