JP5268424B2 - X-ray CT system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus which allows X-rays per cycle of scanning to give an appropriate X-ray dose transmitted through a subject and is suppressed to have an X-ray dose not transmitted through the subject which does not exceed a predetermined value, thereby to improve the quality of an image of the subject. <P>SOLUTION: The apparatus includes a wedge filter disposed between an X-ray source and a subject to cover a fan angle and transmitting X-rays at a determined attenuation rate. The wedge filter has a thin-walled part formed at the center of the wedge filter corresponding to the center toward the extension of the fan angle and having an X-ray with a short transmission distance, and a thick-walled part formed to continue from the thin-walled part toward the extension and has an X ray with a long transmission distance.The wedge filter is driven such that the X-ray radiated from the X-ray source revolving the subject with the rotation of a rotation part and transmitted through the thick-walled part, may pass through the outside of the subject, and the X-ray radiated from the X-ray source and transmitted through the end of the thin-walled part, may pass the body surface of the subject. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to an X-ray CT apparatus for reconstructing an image of a subject based on a detection signal collected by irradiating the subject with X-rays, and in particular, X-rays between an X-ray source and a subject. The present invention relates to an X-ray CT apparatus provided with a wedge filter to be attenuated.

  The human body thickness differs between the front direction in which the scan angle is 0 ° or 180 °, which is the direction in which the X-ray source is located, and the lateral direction in which the scan angle is 90 ° or 270 °. When scanning at a constant dose, the X-ray detection dose that passes through the subject and is detected by the X-ray detector is high for scans in the front direction with a small body thickness, and for scans in the horizontal direction with a large body thickness. Less. That is, the detected dose varies depending on the scan angle, and the image quality level of the image of the subject is changed.

  In order to eliminate the unevenness of the detected dose, there is a first technique for changing the exposure dose according to the scan angle. In this first technique, the exposure dose is increased at the scanning timing from the lateral direction where the body thickness of the subject increases during one rotation, the decrease in the detected dose is avoided, and the image quality level of the subject image Is kept constant. At the same time, at the timing of scanning from the front direction in which the body thickness of the subject decreases during one rotation, the exposure dose is reduced and unnecessary exposure is reduced.

  On the other hand, in order to adjust the X-ray dose distribution in the fan angle direction, a plurality of types of wedge filters are used.

  When scanning the subject from the front direction, a wedge filter with a thin overall plate thickness in which the X-ray dose distribution in the fan angle direction is wide and has a small height difference is suitable. However, when the subject is scanned from the lateral direction using this wedge filter, the X-rays other than the central portion in the fan angle spreading direction reach the X-ray detection unit without passing through the subject, In addition, when scanning a subject with a high exposure dose from the lateral direction, the X-ray dose that has reached the X-ray detection unit without passing through the subject is reflected by the A / D conversion unit used thereafter. The limit value of the dynamic range may be exceeded, causing overflow.

  On the other hand, when scanning the subject from the lateral direction, a wedge filter with a narrow central plate thickness and a thick plate thickness at both ends, which has a narrow range of X-ray dose distribution in the fan angle direction and a large height difference, is used. Is suitable. However, when the subject is scanned from the front direction using this wedge filter, the X-ray attenuation rate at both ends in the spreading direction suddenly becomes higher than the X-ray attenuation rate at the center in the fan angle spreading direction. When scanning from the front direction of a subject with a low exposure dose, the detected dose of X-rays transmitted through both sides of the subject is reduced, and a sufficient image quality level is obtained for images on both sides of the subject. There is a risk of not being able to.

  Since the body thickness of the subject in the fan angle direction generally changes continuously during one rotation, in order to keep the image quality level of the subject image constant, the subject transmission surface during one rotation. Requires a second technique for continuously changing the X-ray dose distribution.

  In relation to the second technique, in order to change the X-ray dose distribution on the subject transmission surface during one rotation, a technique of replacing the wedge filter during rotation is also conceivable. There is a problem that the dose distribution does not change continuously.

  Further, there are the following techniques related to the second technique. A wedge filter that is an attenuator is provided between the X-ray source and the subject. The X-ray emission surface of the wedge filter has a concave cylindrical curved surface. By moving the wedge filter in accordance with the rotation of the X-ray source or the like, the central portion of the wedge filter (the portion where the plate thickness is thin) is positioned on the line connecting the X-ray source and the body axis center of the subject. This is an apparatus in which the intensity of X-rays incident on the line detection unit falls within the dynamic range of the X-ray detection unit (for example, Patent Document 1).

JP 2007-319340 A

  However, in the X-ray CT apparatus described in Patent Document 1, the central portion of the wedge filter is positioned on a straight line connecting the X-ray source and the body axis center of the subject according to the rotation of the X-ray source or the like. Therefore, in the case of a subject having a circular tomographic image shown in FIGS. 5 and 6 of the publication, even when X-rays are irradiated from the lateral direction of the subject, the dose of X-rays passing through the body surface of the subject is high. The A / D conversion unit used thereafter falls within the dynamic range. However, since the subject often does not have a circular tomographic image, X-ray detection is performed while the X-ray passing through the body surface of the subject is not sufficiently attenuated by the wedge filter in one rotation of the X-ray source or the like. As a result, the dose of X-rays passing through the body surface of the subject exceeds the limit value of the dynamic range for the A / D converter, which may cause overflow. .

  The present invention solves the above-described problem. In one rotation scan, the X-ray dose that passes through the subject is made appropriate, and the X-ray dose that does not pass through the subject does not exceed a predetermined value. An object of the present invention is to provide an X-ray CT apparatus capable of improving the image quality of an image of a subject by suppressing the image quality.

In order to solve the above-mentioned problems, the present invention changes the X-ray dose detected by the X-ray detector when the wedge filter that transmits X-rays is provided with a thin part and a thick part and the wedge filter is moved. Focused on being possible.
Specifically, in the first embodiment of the present invention, an X-ray source having a fan angle, and X-ray detection that detects X-rays irradiated from the X-ray source and transmitted through the subject and outputting a detection signal And an image reconstruction unit for reconstructing an image based on the digital signal obtained by converting the detection signal, and the fan angle between the X-ray source and the subject so as to cover a predetermined attenuation. A wedge filter that transmits X-rays at a rate, wherein a thin-walled portion having a short X-ray transmission distance is formed at a central portion of the wedge filter corresponding to a central portion in the fan angle spreading direction. A wedge filter in which a thick part with a long distance is continuously formed from the thin part in the spreading direction; a rotating part that supports the X-ray source and the wedge filter and is rotatable around the subject; With the rotation of the rotating part The X-ray irradiated from the X-ray source that goes around the subject and transmitted through the thick part passes through the outside of the subject, is irradiated from the X-ray source, and is adjacent to the thick part. A drive unit that moves the wedge filter so that X-rays that pass through the end of the thin part pass through the body surface of the subject, and the drive unit transmits the X-rays through the wedge filter. as X-rays transmitted through the end of Rutotomoni moving the wedge filter in the radial direction the thin portion is the direction passes body of the subject that, the wedge filter the X-ray the wedge filter The thinned portion is moved in an axial direction which is a direction orthogonal to a transmitting cross section, and the thinned portion gradually narrows in an axial direction which is a direction orthogonal to a cross section where X-rays pass through the wedge filter. Further has a width of direction, formed is possible to reduce the thickness of the thin portion is an X-ray CT apparatus according to claim.

According to the first aspect of the present invention, by moving the wedge filter in the radial direction in accordance with the rotation of the rotating part, the X-rays that have passed through the thick part of the wedge filter pass outside the subject, Since X-rays that have passed through the edge of the thin portion of the wedge filter pass through the body surface of the subject, the X-ray dose that passes through the subject is made appropriate in one rotation scan and does not pass through the subject. The X-ray dose is suppressed so as not to exceed a predetermined value, and the image quality of the subject image can be improved. In addition, the wedge filter can be formed relatively easily by forming the wedge filter so as to reduce the thickness of the thin portion as the thin portion of the wedge filter is gradually narrowed in the axial direction.

[First Embodiment]
(Constitution)
The configuration of the X-ray CT apparatus according to the first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the configuration of the X-ray CT apparatus.

  In the X-ray CT apparatus, an X-ray source 10 which is an X-ray tube and an X-ray detection unit 20 rotate together around the subject P as one body, and a large number of detection elements are arrayed in a ring shape. There are various types such as a type in which only the X-ray source 10 rotates around the subject P. The present invention is applicable to any type.

  Hereinafter, the X-ray source 10 and the X-ray detection unit 20 will be described as a type that rotates together. A gantry (not shown) is provided with a rotating unit 44 that can rotate around the subject P. The rotation drive unit 43 receives the control signal from the control unit 50 and rotates the rotation unit 44. An X-ray source 10 and an X-ray detection unit 20 are supported on the rotation unit 44.

  The subject P is placed on a top plate (not shown) so as to be movable in the body axis direction along the rotation center axis of the rotating unit 44 and movable in the vertical direction. The top board drive unit 42 receives the control signal from the control unit 50 and moves the top board in a predetermined direction.

  A high voltage is applied from the high voltage generator 11 between the cathode and the anode of the X-ray source 10, and a tube current is supplied from the high voltage generator 11 to the X-ray source 10. The high voltage generator 11 receives a control signal from the control unit 50, changes the tube current supplied to the X-ray source 10, and adjusts the X-ray dose generated from the X-ray source 10.

  X-rays irradiated from the X-ray source 10 pass through the subject P and enter the X-ray detection unit 20. The X-ray detection unit 20 outputs a detection signal based on the X-ray dose transmitted through the subject P. The X-ray source 10 has a fan angle. Here, the fan angle refers to an X-ray spread in a direction orthogonal to the rotation center axis of the X-ray source 10.

  The mechanism of converting incident X-rays into electric charges in the X-ray detection unit 20 is to convert X-rays into light by a phosphor such as a scintillator 21 and further to convert the light into an electrical signal by a photoelectric conversion element such as a photo-die auto 22. Convert to The electric signal is amplified by the signal amplifying unit 23, and the amplified electric signal (detection signal) is converted into a digital signal by the A / D conversion unit 24. The image reconstruction unit 31 reconstructs an image based on the detection signal and outputs the image data to the display unit 32.

  In the first embodiment, the A / D converter 24 converts an analog voltage signal within a certain range into 16-bit digital detection data. That is, in other words, the dynamic range that is the width of the value of one pixel of the display unit 32 is 16 bits. However, when the voltage signal exceeds a certain range in which A / D conversion is possible, the A / D conversion unit 24 becomes saturated and overflows. The range of the certain size is a dynamic range that the A / D converter 24 can convert linearly.

  In order to reconstruct one slice of tomographic image data, the full reconstruction method requires projection data for approximately 360 ° around the subject P for about 360 °. Note that the half reconstruction method also requires projection data corresponding to the fan angle added to 180 °.

  The rotating part 44 is provided with a wedge filter 60 so as to be movable. The wedge filter 60 is configured to be rotatable around the subject P together with the X-ray source 10 by being provided in the rotating unit 44. The wedge filter 60 is provided between the X-ray source 10 and the subject P and has an attenuation factor for attenuating X-rays.

  In the first embodiment, the drive unit 41 receives the control signal from the control unit 50 and moves the wedge filter 60 in the radial direction, which is the direction in which X-rays pass through the wedge filter 60. The radial direction corresponds to the plate thickness direction of the wedge filter 60.

  Next, the wedge filter 60 will be described with reference to FIG. 2A, 2 </ b> B, 2 </ b> C, and 2 </ b> D are a perspective view, a front view, a side view, and a plan view of the wedge filter 60. In the description of the wedge filter 60, the fan angle spreading direction is the width direction of the wedge filter 60. Further, the axial direction is defined as the width direction of the wedge filter 60 and the direction orthogonal to the radial direction in which the above-described X-rays are transmitted through the wedge filter 60. Therefore, the axial direction refers to a direction orthogonal to the cross section through which X-rays pass through the wedge filter 60. The axial direction corresponds to the depth direction of the wedge filter 60.

  The wedge filter 60 is formed in a substantially rectangular parallelepiped. The entrance surface 61 of the wedge filter 60 which is the surface on the side receiving X-rays from the X-ray source 10 is formed in a concave shape. The exit surface 62 of the wedge filter, which is the surface on the side that emits X-rays toward the subject P, is formed flat. A wedge filter 60 in which the incident surface 61 is formed in a concave shape is shown in FIG.

  By forming the incident surface 61 in a concave shape, a thin portion 63 having a short X-ray transmission distance is formed in the central portion of the wedge filter 60 corresponding to the central portion in the fan angle spreading direction. A thick portion 64 having a long X-ray transmission distance is formed at both ends of the wedge filter 60 continuously from the thin portion 63 in the spreading direction. The thin portion 63 is shown in FIG. 2B as a region with a small curvature (between B and C).

  FIGS. 2B and 2D show the thin portion 63 in which the region (between B and C) of the thin portion 63 is constant in the axial direction. The ends of the thin portion 63 are adjacent to the thick portion 64 at points B and C, respectively. FIG. 2C shows the thin portion 63 in which the thickness t of the thin portion 63 is constant in the axial direction.

  The end of the thin portion 63 is a part including the point B or the point C. The end portion of the thin portion 63 passes through the end portion of the thin portion 63, and an A / D conversion in which a detection signal obtained by detecting X-rays passing through a body surface tangent path in contact with the body surface of the subject P is used thereafter. The plate 24 has a thickness that does not exceed a predetermined value that causes an overflow when converted into a digital signal by the unit 24.

  Strictly speaking, the positions of the points B and C, which are the ends of the thin-walled portion 63, differ depending on the tomographic shape of the subject P that passes through X, and therefore, depending on the type of tomographic shape of the subject P. It is necessary to prepare the wedge filter 60. However, the tomographic shape of the subject P covers other types, and it is not practical to prepare the wedge filter 60 corresponding to the shape. Therefore, the wedge filter 60 corresponding to the predetermined tomographic shapes of a plurality of types of subjects P is prepared. The positions of the points B and C, which are the ends of the thin portion 63, are positions corresponding to the tomographic shapes of the plural types of subjects P.

  In the first embodiment, the X-rays passing through the body surface tangent path of the subject P are shown to pass through the end portion of the thin portion 63, but the thin portion is not limited to the end portion of the thin portion 63. The central part 63 may be sufficient, and the thick part 64 side adjacent to the thin part 63 may be sufficient.

  Next, how to provide the thin portion 63 and the thick portion 64 of the wedge filter 60 will be described with reference to FIG. FIG. 3 is an operation explanatory diagram showing the operation of the wedge filter. The body thicknesses in the front direction and the lateral direction of the subject P are respectively measured from the dual scan images, and the fan angle α necessary for FOV (field of view) in the scan from the front direction of the subject P is calculated. The fan angle α is shown in FIG. Similarly, the fan angle β required for FOV in the scan from the lateral direction of the subject P is calculated. The fan angle β is shown in FIG.

  The radial positions r1 and r2 of the wedge filter 60 are calculated such that the fan angle α required for the FOV falls within the region of the thin portion 63 (between B and C). Further, the positions r1 and r2 are shown in FIGS. 3 (a) and 3 (b). The wedge filter 60 is controlled by the control unit 50 with the same parameters as the control of the rotating unit 44, and reciprocates between r1 and r2 according to the scan angle.

  The width in the axial direction of the wedge filter 60 can cover the cone angle of the X-ray irradiated from the X-ray source 10 in the wedge filter 60 moved to the radial position r2 farthest from the X-ray source 10. It becomes width.

  The emission surface 62 of the wedge filter 60 may be formed in a concave shape, and the thin portion 63 and the thick portion 64 may be provided. The thin portion 63 is formed in a shape corresponding to the tomographic shape of the subject P that transmits X-rays. Further, the thin wall portion 63 and the thick wall portion 64 may be provided by allowing the wedge filter 60 to pass through, for example, a hole having a substantially elliptical cross section in the axial direction.

  The relationship between the radial movement of the wedge filter 60 and the transmission distance of X-rays that pass through the wedge filter 60 is as follows. When X-rays pass through the wedge filter 60 away from the X-ray source 10, compared to the transmission distance of X-rays that pass through the thin portion 63 that is the central portion of the wedge filter 60 corresponding to the central portion in the fan angle direction, The transmission distance of X-rays transmitted through the thick wall portions 64 that are both ends of the wedge filter 60 corresponding to both ends in the fan angle direction is increased, and the X-ray dose distribution on the transmission surface of the subject P is at the center. It is high at a certain thin-walled portion 63 and low at thick-walled portions that are both end portions.

  When X-rays pass through the wedge filter 60 close to the X-ray source 10, the transmission distance through which the X-rays pass through the thin portion 63 over the fan angle direction is substantially uniform, and the X-ray dose distribution on the transmission surface of the subject P Becomes flat. Note that the X-ray attenuation near the center of the thin portion 63 does not change regardless of whether the wedge filter 60 is moved away from or closer to the X-ray source 10.

  The driving unit 41 is irradiated from the X-ray source 10 that travels around the subject P in response to the rotation of the rotating unit 44, and X-rays that pass outside the subject P pass through the thick part 64, The wedge filter 60 is driven so that X-rays irradiated from the radiation source 10 and passing through the body surface tangent path that contacts the body surface of the subject P pass through the end of the thin portion 63.

  Next, the movement of the wedge filter 60 will be described with reference to FIG. The distance between the X-ray source and the wedge is r1 when the abdomen of the subject P is scanned from the front direction (scan angles are 0 ° and 180 °). The distribution of X-ray dose over a wide range is shown in FIG. Also, r2 when the abdomen of the subject P is scanned from the lateral direction (scan angles are 90 °, 270 °). The X-ray dose distribution over a narrow range is shown in FIG.

  Next, the dose of X-rays irradiated from the X-ray source 10 will be described with reference to FIGS. 3 and 4A. The relationship between the scan angle and the tube current is shown by a curve in FIG. The control unit 50 sends a control signal to the high pressure generator 11 in accordance with the rotation of the rotating unit 44. The high pressure generator 11 receives the control signal and changes the dose of X-rays emitted from the X-ray source 10. In FIG. 4A, when the abdomen of the subject P is scanned from the front direction (scan angles are 0 ° and 180 °), the tube current is small and the dose of X-rays emitted from the X-ray source 10 is reduced. ing. On the contrary, when the abdomen of the subject P is scanned from the lateral direction (scan angles are 90 °, 270 °), the tube current is large and the dose of X-rays irradiated from the X-ray source 10 is increased. The control unit 50 may control the high pressure generator 11 in accordance with the movement of the wedge filter 60.

  FIGS. 3A and 3B show the X-ray dose Smax passing through the body surface tangent path of the subject P. FIG. When the detection signal based on the X-ray dose Smax is converted into digital data by the A / D converter 24 thereafter, the detection signal becomes a value that does not exceed a predetermined value that causes an overflow.

  Next, the operation of the wedge filter 60 will be described with reference to FIG. The drive unit 41 adjusts the moving speed of the wedge filter 60. The relationship between the scan angle and the distance between the X-ray source and the wedge filter is shown by a curve in FIG. In FIG. 4B, the moving speed of the wedge filter 60 is indicated by the slope of the curve. Further, the acceleration of the wedge filter 60 is indicated by the rate of change of inclination. For example, the acceleration of the wedge filter 60 is large before and after scanning the abdomen of the subject P from the lateral direction (scanning angle is 90 °, 270 °). On the contrary, the acceleration of the wedge filter 60 is small before and after scanning the abdomen of the subject P from the front direction (scanning angle is 0 °, 180 °).

(Operation)
In the scan of the subject P by the X-ray CT apparatus, the high-voltage generator 11 adjusts the tube current supplied to the X-ray source 10 according to the rotation (scan angle) of the rotating unit 44. The magnitude of the tube current to be supplied is shown in FIG. Further, the drive unit 41 reciprocates the wedge filter 60 in the radial direction according to the rotation (scan angle) of the rotation unit 44. The movement position of the wedge filter 60 is based on the distance between the X-ray source and the wedge shown in FIG.

  The X-ray dose is adjusted according to the rotation of the rotating unit 44. Further, the wedge filter 60 reciprocates between the X-ray source 10 and the subject P according to one rotation of the rotating unit 44. At this time, the drive unit 41 adjusts the moving speed of the wedge filter 60. Thereby, the X-rays that have passed through the thin-walled portion 63 pass through the subject P, so that the X-ray dose is made appropriate. Further, since the X-ray transmitted through the thick part 64 passes outside the subject P, the X-ray dose is suppressed so as not to exceed a predetermined value. Furthermore, since the X-ray transmitted through the end of the thin portion 63 passes through the body surface tangent path of the subject P, the X-ray dose is made appropriate. When the X-ray dose becomes appropriate, the detection signal of the X-ray detection unit becomes a numerical value that does not cause overflow when converted into digital data thereafter, and the image of the subject reconstructed based on the digital data The image quality can be improved.

[Second Embodiment]
Next, an X-ray CT apparatus according to a second embodiment of the present invention will be described with reference to FIGS. FIG. 5 is an explanatory diagram showing the wedge filter 60. FIG. 6 is an operation explanatory diagram showing the operation of the wedge filter 60. In the second embodiment, only two configurations different from the X-ray CT apparatus according to the first embodiment will be described, and the description of the same configuration will be omitted.

  A first configuration different from the first embodiment will be described with reference to FIG. In the wedge filter 60 according to the first embodiment, the region (between B and C) that is the width in the fan angle spreading direction of the thin portion 63 is constant in the axial direction, whereas in the second embodiment, The region of the thin portion 63 (between B and C) gradually changes in the axial direction. FIG. 5B shows a wedge filter 60 in which the region (between B and C) of the thin portion 63 is gradually narrowed in the axial direction (A1 direction).

  As described above, the region (between B and C) of the thin portion 63 changes, but the plate thickness t of the thin portion 63 is constant with respect to the axial direction as in the first embodiment. FIG. 5C shows a thin portion 63 having a constant plate thickness t.

  As in the first embodiment, the thin wall portion 63 and the thick wall portion 64 may be provided by forming the emission surface 62 of the wedge filter 60 into a concave shape, and by passing a hole through the wedge filter 60. It may be provided.

  A second configuration different from the first embodiment is the operation of the wedge filter 60. The operation of the wedge filter 60 will be described with reference to FIG. The drive unit 41 receives the control signal from the control unit 50 and moves the wedge filter 60 in the axial direction in accordance with the rotation of the rotation unit 44.

  When the abdomen of the subject P is scanned from the front direction (scan angles are 0 ° and 180 °), the wedge filter 60 moves in the A1 direction in FIG. Thereby, the focal point of the X-ray comes to be located in a portion where the thin portion 63 is wide. This wide portion is a portion having a small curvature, and when X-rays are transmitted, the X-ray transmission distance is uniform from the center to both ends in the fan angle direction, and the X-ray dose distribution on the transmission surface of the subject P Becomes flat. The X-ray dose distribution at this time is shown in FIG.

  Further, when the abdomen of the subject P is scanned from the lateral direction (scan angles are 90 °, 270 °), the wedge filter 60 moves in the A2 direction in FIG. Thereby, the focal point of the X-ray comes to be located in a portion where the region of the thin portion 63 is narrow. Both sides of the narrow portion are thick portions 64. When X-rays pass through a narrow part of the thin part 63, the X-ray transmission distance is short, and when X-rays pass through the thick part 64, the X-ray transmission distance becomes long. The dose distribution of X-rays on the transmission surface of the subject P has a large difference in height between the central part where the X-rays pass through the thin part 63 and both end parts where the X-rays pass through the thick part 64. The X-ray dose distribution at this time is shown in FIG.

  Note that the X-ray dose transmitted through the thin portion 63 shown in FIG. 6B is larger than the X-ray dose transmitted through the thin portion 63 shown in FIG. This is because the tube current when scanning the abdomen of the subject P from the lateral direction is made larger than the tube current when scanning the abdomen from the front direction, as in the first embodiment.

  In the first embodiment, the wedge filter 60 is moved in the radial direction. In the second embodiment, the wedge filter 60 is not moved in the radial direction, and the distance r between the X-ray source 10 and the wedge filter 60 is as follows. Always constant.

  With the above configuration, the X-ray CT apparatus according to the second embodiment is suitable when a space for moving the wedge filter 60 in the radial direction cannot be secured.

  In the second embodiment, the driving unit 41 also emits X-rays that are irradiated from the X-ray source 10 that travels around the subject P and passes outside the subject P in response to the rotation of the rotating unit 44. The wedge filter 60 is transmitted so that X-rays transmitted through the thick part 64 and irradiated from the X-ray source 10 and passing through the body surface tangent path contacting the body surface of the subject P pass through the end part of the thin part 63. Move. The X-ray dose passing through the body surface tangent path of the subject P is also Smax shown in FIGS. 6A and 6B, and the detection signal based on the X-ray dose Smax is a subsequent A / D conversion unit. When converted to digital data at 24, the value does not exceed a predetermined value that would cause overflow.

[Third Embodiment]
Next, an X-ray CT apparatus according to a third embodiment of the present invention will be described with reference to FIGS. FIG. 7 is an explanatory diagram showing the wedge filter 60. FIG. 8 is an operation explanatory diagram showing the operation of the wedge filter 60. In the third embodiment, only the configuration different from the X-ray CT apparatus according to the second embodiment will be described, and the description of the same configuration will be omitted.

  A first configuration different from the second embodiment will be described with reference to FIG. In the second embodiment, the thickness t of the thin wall portion 63 of the wedge filter 60 is constant, whereas in the third embodiment, the thickness of the thin wall portion 63 is in the fan angle spreading direction of the thin wall portion 63. The width region (between B and C) decreases as it narrows in the axial direction. FIG. 7C shows a wedge filter 60 in which the thickness of the thin portion 63 decreases from t1 to t2 axial direction (A1 direction) by increasing the depth of the concave shape formed on the incident surface 61.

  When the wedge filter 60 moves in the A1 direction, a wide portion of the thin portion 63 (between B and C) is positioned on the line in the X-ray irradiation direction (radial direction), and the wedge filter 60 moves in the A2 direction. 7A and 7B show the wedge filter 60 in which the narrow portion of the region (between B and C) of the thin portion 63 is positioned on the line in the X-ray irradiation direction (radial direction).

  As in the first embodiment and the second embodiment, the thin wall portion 63 and the thick wall portion 64 may be provided by forming the emission surface 62 of the wedge filter 60 into a concave shape. You may provide by penetrating.

  The second configuration different from the second embodiment is that the high-pressure generator 11 does not change the rotation of the rotating unit 44 and keeps the X-ray dose emitted from the X-ray source 10 unchanged. In the point. The X-ray dose does not need to be adjusted according to the rotation of the rotating unit 44, and the control of the high-pressure generator 11 is simplified.

  As in the second embodiment, the X-ray dose that passes through the thin portion 63 shown in FIG. 8B is larger than the X-ray dose that passes through the thin portion 63 shown in FIG. This is because when the abdomen of the subject P is scanned from the lateral direction as shown in FIG. 8A, the thickness of the thin portion 63 of the wedge filter 60 through which X-rays pass is reduced to t2. This is due to the increased dose of radiation. On the contrary, when the abdomen of the subject P is scanned from the lateral direction as shown in FIG. 8B, the plate thickness of the thin portion 63 is increased to t1, so that the X-ray dose transmitted is reduced.

  In the third embodiment, as the region (between B and C) of the thin portion 63 gradually narrows in the axial direction, the thin portion 63 of the wedge filter 60 is formed into a concave depth formed on the incident surface 61. By increasing the thickness, the plate thickness was reduced from t1 to t2 in the axial direction, but by forming the exit surface 62 on a slope, the plate thickness of the thin portion 63 was reduced from t1 to t2. May be. FIG. 9 shows a wedge filter 60 in which the emission surface 62 is formed on a slope. FIG. 9 is an explanatory view showing another example of the wedge filter, and FIGS. 9A, 9B, 9C, and 9D are a perspective view, a front view, a side view, and a plan view of the wedge filter 60. FIG. . According to the wedge filter 60 according to another example, the shape of the incident surface 61 formed in a concave shape does not need to be complicated, and the wedge filter 60 can be formed relatively easily.

  In the above embodiment, the wedge filter 60 is integrally formed. However, for example, when the shape of the thin portion 63 and the thick portion 64 is complicated, it is difficult to form the wedge filter 60. The wedge filter 60 may be formed by combining a plurality of members.

  Further, in the above-described embodiment, the X-ray detection signal passing through the body surface tangent path of the subject P is converted into subsequent digital data by adjusting the X-ray dose or moving the wedge filter. In some cases, the limit value does not exceed a predetermined value that does not cause overflow, but may be set to a numerical value that is equal to or less than the limit value.

1 is a block diagram showing a configuration of an X-ray CT apparatus according to a first embodiment of the present invention. It is explanatory drawing which shows the wedge filter which concerns on 1st Embodiment of this invention. It is operation | movement explanatory drawing which shows operation | movement of the wedge filter which concerns on 1st Embodiment of this invention. (A) is explanatory drawing which shows the relationship between the scan angle and tube current which concerns on 1st Embodiment of this invention, (b) is explanatory drawing which shows the relationship between a scan angle and the distance between X-ray source-wedge filters. It is. It is explanatory drawing which shows the wedge filter which concerns on 2nd Embodiment of this invention. It is operation | movement explanatory drawing which shows operation | movement of the wedge filter which concerns on 2nd Embodiment of this invention. It is explanatory drawing which shows the wedge filter which concerns on 3rd Embodiment of this invention. It is operation | movement explanatory drawing which shows operation | movement of the wedge filter which concerns on 3rd Embodiment of this invention. It is explanatory drawing which shows the other example of the wedge filter which concerns on 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 X-ray source 11 High voltage generator 20 X-ray detection part 21 Scintillator 22 Photodiode 23 Signal amplifier 24 A / D conversion part 31 Image reconstruction part 32 Display part 41 Drive part 42 Top plate drive part 43 Rotation drive part 44 Rotation part 50 Control Unit 60 Wedge Filter 61 Incident Surface 62 Outgoing Surface 63 Thin Wall Portion 64 Thick Wall Portion

Claims (3)

An X-ray source having a fan angle;
An X-ray detector that detects X-rays irradiated from the X-ray source and transmitted through the subject, and outputs a detection signal;
An image reconstruction unit for reconstructing an image based on the digital signal obtained by converting the detection signal;
A wedge filter that is disposed between the X-ray source and the subject so as to cover the fan angle and transmits X-rays with a predetermined attenuation rate. A wedge filter formed in a central portion of the wedge filter corresponding to a central portion in a widening direction of the corner, wherein a thick wall portion having a long transmission distance of the X-ray is continuously formed in the widening direction from the thin wall portion;
A rotating unit that supports the X-ray source and the wedge filter and is rotatable around the subject;
The X-ray source that irradiates around the subject as the rotating unit rotates, and the X-ray that has passed through the thick-walled portion passes outside the subject and is irradiated from the X-ray source. A drive unit that moves the wedge filter so that X-rays passing through the end of the thin part adjacent to the thick part pass through the body surface of the subject;
Have
The drive unit, such that the X-rays the X-rays transmitted through the end of the Rutotomoni the thin portion moving the wedge filter in the radial direction which passes through the wedge filter passes body of the subject In addition, the wedge filter is moved in an axial direction which is a direction orthogonal to a cross section through which the X-ray passes through the wedge filter,
The thin portion further has a width in the spreading direction that gradually narrows in an axial direction, which is a direction orthogonal to a cross section through which X-rays pass through the wedge filter, so as to reduce the thickness of the thin portion. Rukoto is formed,
X-ray CT apparatus characterized by this.   The end portion of the thin portion adjacent to the thick portion overflows when an X-ray detection signal transmitted through the end portion of the thin portion and transmitted through the body surface of the subject is converted into the digital signal. The X-ray CT apparatus according to claim 1, wherein the X-ray CT apparatus has a plate thickness that does not exceed a predetermined value that causes the occurrence of the above.   The said drive part adjusts the moving acceleration of the said wedge filter according to the rotation angle of the said X-ray source and the said wedge filter by the said rotation part, The Claim 1 or Claim 2 characterized by the above-mentioned. X-ray CT system.

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