JP4619704B2 - X-ray computed tomography system - Google Patents

Description

  The present invention relates to an X-ray computed tomography apparatus that limits an X-ray irradiation field to a region of interest of a subject.

  Irradiating X-rays limited to the region of interest is one of the most effective methods for reducing exposure. However, this method inevitably causes artifacts in the reconstructed image. This occurs because the projection data set of one view includes a part of information outside the region of interest, but the information of the part is not included in the projection data set of another view.

  In Patent Documents 1 and 2, the occurrence of artifacts is suppressed by supplementing data outside the region of interest with data measured in advance.

However, the difference in data acquisition time causes body movement artifacts. Therefore, it has been difficult to achieve both the prevention of image quality deterioration and the reduction of exposure.
Japanese Patent Laid-Open No. 10-248835 Japanese Patent Laid-Open No. 11-28202

  An object of the present invention is to achieve both prevention of image quality deterioration and reduction of exposure.

X-ray computer tomography apparatus according to the present invention, the X-ray tube for generating X-rays, said a multichannel X-ray detector which detects X-rays transmitted through the object and generates projection data, wherein A rotating mechanism that supports the X-ray tube and the X-ray detector so as to be rotatable around the subject, and a region of interest in the subject is irradiated with X-rays at a relatively high dose, and the region other than the region of interest is irradiated. In order to irradiate X-rays at a relatively low dose, a translucent collimator plate that is movably provided, a range in which X-rays are irradiated at a relatively high dose during rotation of the X-ray tube, and X at a relatively low dose A collimator controller that controls the position of the semi-transmissive collimator plate to dynamically change the range of irradiation of the radiation, a first reference detector that detects a relatively high dose of X-ray, A second reference detector for detecting a low-dose X-ray dose; Depending on the position of the semi-transmissive collimator plate controlled by a collimator controller, the first channel of the X-ray detector corresponding to the range of the X-ray irradiation with the relatively high dose and the X with the relatively low dose Distinguishing from the second channel of the X-ray detector corresponding to the irradiation range of the X-ray, and correcting the projection data of the first channel based on the X-ray dose detected by the first reference detector And a data correction unit for correcting the projection data of the second channel based on the X-ray dose detected by the second reference detector, and the X-ray dose detected by the first reference detector. and projection data corrected on the basis, an image reconstruction unit which reconstructs image data on the basis of the corrected projection data based on the dose of X-rays detected by said second reference detector Comprising.

  According to the present invention, both image quality deterioration prevention and exposure reduction can be achieved.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. As is well known, the X-ray computed tomography apparatus has a rotation / rotation (ROTATE / ROTATE) type in which an X-ray tube and a radiation detector are rotated as one body, and a large number of detection elements in a ring shape. There are various types such as a fixed / rotation (STATIONARY / ROTATE) type in which only the X-ray tube rotates around the subject, and the present invention can be applied to any type. Here, the rotation / rotation type that currently occupies the mainstream will be described. In addition, to reconstruct one slice of tomographic image data, projection data for about 360 ° around the periphery of the subject is required, and projection data for 180 ° + view angle is also required in the half scan method. . The present invention can be applied to any reconstruction method. Here, the half scan method will be described as an example. In addition, the mechanism for converting incident X-rays into electric charges is based on an indirect conversion type in which X-rays are converted into light by a phosphor such as a scintillator, and the light is further converted into electric charges by a photoelectric conversion element such as a photodiode. The generation of electron-hole pairs in semiconductors and their transfer to the electrode, that is, the direct conversion type utilizing a photoconductive phenomenon, is the mainstream. Any of these methods may be adopted as the X-ray detection element.

(First embodiment)
FIG. 1 shows the configuration of the main part of an X-ray computed tomography apparatus according to the first embodiment. The X-ray tube 11 is mounted on a ring-shaped rotating frame (not shown) together with the multi-channel X-ray detector 13. An upper collimator 15 for limiting the X-ray irradiation field to the subject P is attached to the X-ray tube 11. As shown in FIGS. 2 and 3, the upper collimator 15 typically has four collimator plates having the property of not transmitting X-rays generated from the X-ray tube 11, that is, the property of completely shielding the X-rays. 33, 35, 37, 39 are provided.

  The upper collimator 15 typically has two collimator plates 33, 35, 37, and 39 and has a property of transmitting a part of X-rays generated from the X-ray tube 11 (semi-transmissive). Collimator plates 41 and 43 are provided. When the X-ray transmittance is defined as the ratio (Iout / Iin) of the transmitted X-ray dose Iout to the incident X-ray dose Iin, the four collimator plates 33, 35, 37, and 39 are zero or an approximation thereof. It has an X-ray transmittance indicating a value. On the other hand, the two collimator plates 41 and 43 have an X-ray transmittance higher than that of the four collimator plates 33, 35, 37 and 39.

  The four collimator plates 33, 35, 37, 39 are typically made of lead or an alloy containing lead having a thickness necessary to obtain zero or an approximate value of the X-ray transmittance. On the other hand, the two semi-transmissive collimator plates 41 and 43 are made of, for example, Mo having a lower attenuation coefficient than lead. However, the two semi-transmissive collimator plates 41 and 43 may be made of the same lead or an alloy containing lead as the collimator plates 33, 35, 37 and 39. In this case, the collimator plates 41 and 43 are thinner than the collimator plates 33, 35, 37 and 39.

  The collimator plates 33 and 35 are supported so as to be individually movable with respect to the slice direction (Z). The collimator plates 33 and 35 determine the X-ray width (slice thickness) in the slice direction. The collimator plates 37 and 39 are supported so as to be individually movable with respect to the channel direction (X). The collimator plates 37 and 39 determine the X-ray spread angle (fan angle) α in the channel direction.

  Typically, the collimator plates 37 and 39 are fully opened corresponding to the full width of the X-ray detector 13. The upper collimator 15 may not include the collimator plates 37 and 39.

  The semi-transmissive collimator plates 41 and 43 are supported so as to be individually movable with respect to the channel direction (X). Further, the movement of the semi-transmissive collimator plates 41 and 43 is completely independent of the movement of the collimator plates 37 and 39. The spread angle β of the relatively high dose X-ray is determined by the semi-transmissive collimator plates 41 and 43. On both sides of the relatively high dose range, the subject is irradiated with a relatively low dose of X-rays attenuated by the semi-transmissive collimator plates 41 and 43. The angle width β of the high dose range and the crossing angle (irradiation angle) of the center line of the high dose range with respect to the Y axis are determined by the positions of the collimator plates 41 and 43, respectively.

  X-rays from the X-ray tube 11 are completely shielded by the collimator plates 33, 35, 37 and 39. Therefore, the X-rays pass through the rectangular opening formed by the collimator plates 33, 35, 37, and 39 and illuminate the subject P. X-rays in a part of the X-ray irradiation range are attenuated by the semi-transmissive collimator plates 41 and 43 and irradiated with a low dose. Since the X-rays in the remaining range of the X-ray irradiation range do not pass through the semi-transmissive collimator plates 41 and 43, they are irradiated with a high dose.

  The upper collimator 15 is provided with first and second reference detectors 17 and 19. The first reference detector 17 is disposed between the X-ray tube 11 and the collimator plates 33, 35, 37, 39, 41, 43. The first reference detector 17 detects the X-ray irradiation dose in the high dose range. The second reference detector 19 is disposed between the collimator plate 41 or 43 and the subject. The second reference detector 19 detects the X-ray irradiation dose in the low dose range.

  Returning to FIG. A data acquisition unit 21 generally called a DAS (data acquisition system) converts a signal output from the detector 13 for each channel into a voltage signal, amplifies it, converts it into a digital signal, and converts it into a data signal. (Raw data) is output. The logarithmic conversion unit 22 performs logarithmic conversion on raw data. The data correction unit 23 corrects the logarithmically converted row data based on the outputs of the first and second reference detectors 17 and 19. As is well known, this correction is a process of normalizing data in accordance with the actually detected X-ray dose in order to reduce the data change caused by the X-ray dose fluctuation. The data correction unit 23 corrects the data of the channel corresponding to the high dose range based on the output of the first reference detector 17 and corresponds to the low dose range based on the output of the second reference detector 19. Channel data to be corrected is corrected. Thereby, the difference in the data value caused by the difference in the X-ray dose can be corrected. The channel corresponding to the high dose range and the channel corresponding to the low dose range can be distinguished by the respective positions of the semi-transmissive collimator plates 41 and 43 controlled by the collimator control unit 31. The image reconstruction unit 25 reconstructs image data based on the corrected data. The image is displayed on the screen of the display unit 27. The input device 29 has a keyboard and a pointing device such as a mouse, and is mainly provided for setting a region of interest (ROI) on a displayed image. The collimator control unit 31 controls the upper collimator 15 to adjust the positions of the collimator plates 33, 35, 37, 39, 41, 43. In particular, the collimator control unit 31 determines the relationship between the angle of the X-ray tube 11 and the positions of the semi-transmissive collimator plates 41 and 43 according to the set region of interest (ROI), and X-rays according to the determined relationship. During the rotation of the tube 11, the positions of the collimator plates 41 and 43 are dynamically changed.

  Next, the operation of this embodiment will be described. As shown in FIG. 4, a scanogram is first photographed (S1). As is well known, the X-ray tube 11 is fixed at a specific angle. The top is moved continuously with the subject. X-rays are continuously generated and transmitted X-rays are repeatedly detected. As a result, a scanogram is taken. The scanogram is displayed on the display unit 27 as shown in FIG. A desired slice is set on the scanogram in accordance with an operator command input from the input device 29. A pre-scan is executed on the set slice (S2). In the pre-scan, the collimator control unit 31 controls the collimator plates 37, 39, 41, and 43 with respect to the channel direction to be fully opened, and the positions of the collimator plates 33 and 35 with respect to the slice direction correspond to a predetermined slice thickness for pre-scan. Adjusted. The top plate is stopped at the slice position, and the X-ray tube 11 is rotated. X-rays are continuously generated and transmitted X-rays are repeatedly detected. Thereby, multidirectional projection data is collected. Image data (tomographic image data) is reconstructed based on the multi-directional projection data and displayed on the display unit 27 as shown in FIG. A region of interest (ROI) is set on the tomographic image via the input device 29 (S3). The region of interest (ROI) is set to an ellipse, a polygon, or an arbitrary shape so as to accommodate a desired part in the cross section.

  In the collimator controller 31, as shown in FIG. 1, the positions of the semi-transmissive collimator plates 41 and 43 are set to the X-ray tube so that the X-ray high dose range is limited to the set region of interest (ROI). It is calculated every 11 angles, for example every 5 °. According to this calculation result, the positions of the semi-transmissive collimator plates 41 and 43 are dynamically changed in accordance with the rotation of the X-ray tube 11 in the main scan (S5). Accordingly, the region of interest (ROI) is irradiated with X-rays at a high dose, and the region other than the region of interest (ROI) is irradiated with X-rays at a low dose.

  Data collected by the data collection unit 21 by the main scan is sent to the data correction unit 23 via the logarithmic conversion unit 22. The data correction unit 23 specifies a channel corresponding to the high dose region and a channel corresponding to the low dose region for each angle of the X-ray tube 11 based on the positions of the semi-transmissive collimator plates 41 and 43 ( S6). The data correction unit 23 corrects the data of the channel corresponding to the high dose range based on the output of the first reference detector 17 and corresponds to the low dose range based on the output of the second reference detector 19. The data of the channel to be corrected is corrected (S7). Thereby, the dependence of the data on the X-ray dose is reduced and the data is normalized.

  Based on the corrected high dose range data and low dose range data, the image reconstruction unit 25 reconstructs the image data (S8) and displays it on the screen of the display unit 27.

  According to this embodiment, X-rays are irradiated at a low dose to a part other than the region of interest, so that the exposure can be reduced compared to a case where X-rays are irradiated at a high dose including the part other than the region of interest. Can be planned. In addition, data on the region of interest is collected with high-dose X-rays, and data on regions other than the region of interest is collected with low-dose X-rays in parallel with the data on the region of interest, thus preventing image quality degradation. Can do.

(Second Embodiment)
The effect similar to the said 1st Embodiment is realizable with a 2-tube system. As shown in FIG. 6, for example, first and second X-ray tubes 11-1 and 11-2 are provided at positions shifted by 90 °. First and second X-ray detectors 13-1 and 13-2 are provided at positions facing each other. Upper collimators 15-1 and 15-2 are attached to the first and second X-ray tubes 11-1 and 11-2, respectively. As shown in FIG. 7, the upper collimators 15-1 and 15-2 include four collimator plates 33, 35, 37, and 39 having shielding properties. A semi-permeable collimator plate is not provided. The openings of the upper collimators 15-1 and 15-2 can be completely individually controlled by the collimator control unit 47. A data correction unit 45 is connected to the first and second X-ray detectors 13-1 and 13-2 via data collection units 21-1 and 21-2, respectively. The data correction unit 45 corrects data (dose correction) according to the output of a reference detector (not shown) attached to the upper collimators 15-1 and 15-2.

  Similar to the first embodiment, scanogram imaging and pre-scanning are executed, and a region of interest (ROI) is set on the tomographic image via the input device 29. In the collimator control unit 47, the X-ray tube 11-1 is irradiated with the X-ray from the first X-ray tube 11-1 only for the set region of interest (ROI). The opening of the collimator 15-1 and its center position are calculated. On the other hand, the collimator control unit 47 fully opens the upper collimator 15-2 so that the entire region including the set region of interest (ROI) is irradiated with X-rays from the second X-ray tube 11-2.

  In the main scan, the opening of the upper collimator 15-1 and the center position thereof are dynamically changed as the first X-ray tube 11-1 rotates. The other upper collimator 15-2 is fixed fully open in the channel direction regardless of the rotation of the second X-ray tube 11-2. In the main scan, X-rays are generated at a relatively high dose from the first X-ray tube 11-1. X-rays are generated from the second X-ray tube 11-2 at a relatively low dose. Data collected by the data collection units 21-1 and 21-2 by the main scan is corrected (dose correction) in the data correction unit 45 according to the output of each reference detector.

  The data output from the first X-ray detector 13-1 is valid for the channel corresponding to the region of interest, and the channel corresponding to the region outside the region of interest is invalid, that is, includes attenuation information (absorption coefficient information) of the subject. Not in. The data of the channel corresponding to the outside of the region of interest is output from the second X-ray detector 13-2 via the data collection unit 21-2, and is supplemented with the data subjected to dose correction by the data correction unit 45. A set of projection data over all channels is generated by the compensation, and the image data is reconstructed by the image reconstruction unit 25 based on the data and displayed on the screen of the display unit 27.

  According to this embodiment, X-rays are irradiated at a low dose to a part other than the region of interest, so that the exposure can be reduced compared to a case where X-rays are irradiated at a high dose including the part other than the region of interest. Can be planned. In addition, data on the region of interest is collected with high-dose X-rays, and data on regions other than the region of interest is collected with low-dose X-rays in parallel with the data on the region of interest, thus preventing image quality degradation. Can do.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

  For example, in the first and second embodiments, the data collected by the low-dose X-rays is used for the data of the channel other than the region of interest in parallel with the collection of the data of the region of interest. This effectively prevents image quality degradation. However, when the prevention of image quality deterioration is not critical, only high-dose X-rays are used to collect only the data of the region of interest, and X-rays are not irradiated outside the region of interest, and the channel data is not collected. You may make it supplement the data of the channels other than an area | region with the standard sample data regarding the same site | part as a test | inspection site | part.

1 is a configuration diagram of an X-ray computed tomography apparatus according to a first embodiment of the present invention. FIG. The perspective view of the upper collimator of FIG. Sectional drawing of the upper collimator of FIG. The figure which shows the operation | movement procedure of 1st Embodiment. The figure which shows the example of a screen at the time of ROI setting of S3 of FIG. The block diagram of the X-ray computed tomography apparatus which concerns on 2nd Embodiment of this invention. The perspective view of the upper collimator of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... X-ray tube, 13 ... X-ray detector, 15 ... Upper collimator, 17 ... 1st reference detector, 19 ... 2nd reference detector, 21 ... Data collection part, 22 ... Logarithmic conversion part, 23 ... Data correction 25, an image reconstruction unit, 27, a display unit, 29, an input device, 31 ... a collimator control unit.

Claims (1)

An X-ray tube that generates X-rays;
A multi-channel X-ray detector that detects projection X-rays and generates projection data; and
A rotation mechanism that supports the X-ray tube and the X-ray detector so as to be rotatable around a subject;
A translucent collimator plate movably provided to irradiate a region of interest in the subject with a relatively high dose of X-rays and irradiate a region other than the region of interest with a relatively low dose of X-rays ;
The position of the semi-transmissive collimator plate is controlled in order to dynamically change the range in which the X-ray is irradiated with a relatively high dose and the range in which the X-ray is irradiated with a relatively low dose during rotation of the X-ray tube. A collimator control unit ,
A first reference detector for detecting a relatively high dose of X-ray;
A second reference detector for detecting a relatively low dose of X-ray;
Depending on the position of the semi-transmissive collimator plate controlled by the collimator control unit, the first channel of the X-ray detector corresponding to the range of the X-ray irradiation with the relatively high dose and the relatively low dose. The second channel of the X-ray detector corresponding to the X-ray irradiation range is distinguished, and the projection data of the first channel is based on the X-ray dose detected by the first reference detector. A data correction unit that corrects and corrects the projection data of the second channel based on the X-ray dose detected by the second reference detector;
And projection data corrected based on the dose of X-rays detected by said first reference detector, on the basis of the corrected projection data based on the dose of X-rays detected by said second reference detector An X-ray computed tomography apparatus comprising: an image reconstruction unit configured to reconstruct image data.

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