JP5075490B2 - X-ray CT system - Google Patents

Description

  The present invention relates to an X-ray CT apparatus that corrects an X-ray source shift (that is, focal shift).

  An X-ray CT apparatus is an X-ray tube apparatus that is an X-ray source that irradiates a subject with X-rays, and an X-ray detector that detects an X-ray dose that has passed through the subject as projection data. The tomographic image of the subject is reconstructed using the projection data of a plurality of angles detected by the X-ray detector, and the reconstructed tomographic image is displayed for image diagnosis.

  The X-ray tube device includes a cathode that emits an electron beam, and an anode that generates X-rays by colliding the electron beam emitted from the cathode. The electron beam is accelerated by a high voltage applied between the cathode and the anode, and is converted into X-rays near the surface in the anode when it collides with the anode. Part of the energy of the accelerated electron beam is converted to X-rays, but most of the rest is converted to heat, heating the anode and causing thermal expansion. As a result, the position of the anode surface that substantially becomes the X-ray generation point varies. The fluctuation of the X-ray generation point becomes a measurement error of the projection data acquired by the X-ray detector, and causes an artifact (false image) on the tomographic image obtained after the reconstruction.

When artifacts occur on the tomographic image, the diagnostic performance of the X-ray CT apparatus is remarkably deteriorated, so it is necessary to reduce fluctuations in the X-ray generation point. In Patent Document 1, the amount of change in the X-ray generation point is detected and detected using a shift detector that detects the shift in the X-ray generation point, particularly the shift in the body axis direction (hereinafter referred to as the slice direction) of the subject. Based on the values, the position of the X-ray generator is controlled to reduce fluctuations in the X-ray generation point.

JP 2005-143812 JP

  Incidentally, X-rays generated from the X-ray generator are separated into a range incident on the shift detector and a range other than that. X-rays in the range incident on the shift detector are scattered by the shift detector, and the dose and quality are different from other X-rays, so they are suitable for use in X-rays for detecting projection data. First, it becomes an invalid exposure for the subject. In order to reduce the ineffective exposure of the subject, it is necessary to make the X-ray range incident on the shift detector as narrow as possible.

  However, in the above-mentioned known documents, no consideration is given to narrowing the range of X-rays incident on the shift detector. Further, in the above document, no consideration has been given to scattered rays generated from the substrate portion of the shift detector.

  Accordingly, an object of the present invention is to reduce invalid exposure of a subject in an X-ray CT apparatus including a shift detector.

  In order to solve the above-described problems, the present invention is configured as follows.

  Equipped with an X-ray source that irradiates the subject with X-rays, an X-ray detector that is disposed opposite to the X-ray source and detects X-rays transmitted through the subject, and the X-ray source and the X-ray detector A scanner that rotates around the subject, an image reconstruction device that reconstructs a tomographic image of the subject based on a transmitted X-ray amount detected by the X-ray detector, and a tomogram reconstructed by the image reconstruction device An image display device that displays an image; a scintillator that converts X-rays into light; a photoelectric conversion element that receives light emitted from the scintillator to generate an electrical signal; and a substrate on which the photoelectric conversion element is mounted. A shift detector that detects the shift amount of the position of the X-ray incident on the X-ray detector, and an X-ray that is incident on the X-ray detector by shifting the position of the X-ray source based on the detection value of the shift detector In an X-ray CT apparatus provided with an X-ray shift apparatus that shifts the position of a line, The scintillator has a protrusion that protrudes toward the rotation center of the scanner from the end of the substrate, and the protrusion is located in the X-ray irradiation field.

  As described above, according to the present invention, the range of X-rays incident on the shift detector that detects the shift amount of the X-ray position can be made as narrow as possible, so that the ineffective exposure of the subject can be reduced.

  An X-ray CT apparatus to which the present invention is applied will be described with reference to the drawings.

  FIG. 1 is an overall configuration diagram of an X-ray CT apparatus 1 to which the present invention is applied. The X-ray CT apparatus 1 includes an X-ray source 1 that irradiates a subject 10 with X-rays, an X-ray detector 2 that is disposed opposite to the X-ray source 1 and detects X-rays that have passed through the subject 10, and an X-ray A scanner that includes the source 1 and the X-ray detector 2 and rotates around the subject 10, and an image reconstruction device that reconstructs a tomographic image of the subject 10 based on the transmitted X-ray dose detected by the X-ray detector 2. An image display device that displays a tomogram reconstructed by the image reconstruction device, a shift detector 7 that detects the shift amount of the position of the X-ray incident on the X-ray detector 2, and a detection value of the shift detector 7 And a position control device 9 for controlling the X-ray position based on the above.

  The X-ray source 1 includes a cathode that emits an electron beam and an anode that generates X-rays by colliding the electron beam emitted from the cathode, in accordance with a high voltage applied between the cathode and the anode. Outputs energy X-rays. The X-ray source 1 is supported by the support mechanism 5 and mounted on the scanner.

  X-rays output from the X-ray source 1 are shaped into a fan beam (fan beam) having a predetermined thickness in the slicing direction by the collimator 6 and irradiated onto the subject 10. X-rays transmitted through the subject 10 are detected as projection data by the X-ray detector 2. By rotating a scanner equipped with the X-ray source 1 and the X-ray detector 2, X-ray irradiation to the subject and detection of transmitted X-rays are performed, thereby obtaining projection data at a plurality of angles.

  In the image reconstruction apparatus, a reconstruction process by back projection is performed on the acquired projection data at a plurality of angles, and a tomographic image of the subject 10 is generated. The generated tomographic image is displayed on an image display device and used for image diagnosis.

  The position of the generation point of X-rays output from the X-ray source 1 varies due to thermal expansion of the anode in the X-ray source. The shift detector 7 is for detecting a shift amount caused by a change in the position of the X-ray generation point. The detection value of the shift detector 7 is input to the position control device 9. The position control device 9 is a signal for adjusting the position of the X-ray generation point based on the detection value of the shift detector 7, and a position adjustment device for the X-ray source 1 or a support mechanism for supporting the X-ray source 1. 5 is output to the position adjusting device of the collimator 6. By controlling at least one of these position adjusting devices, it is possible to compensate for variations in the position of the X-ray generation point.

  The X-rays irradiated to the subject 10 include X-rays 3 in the range of the angle α and X-rays 4 in the range of the angle β. The shift detector 7 is irradiated with X-rays 4 in the range of the angle β. Since the X-ray 4 in the range of the angle β is scattered by the shift detector 7, it is not suitable for acquiring projection data of the subject 10, and becomes invalid exposure to the subject 10. In order to reduce the ineffective exposure, it is necessary to make the angle β as narrow as possible.

  2 and 3 show an embodiment of the shift detector 7 for narrowing the angle β. FIG. 2 is a plan view of the shift detector 7, that is, a view seen from the X-ray source 1 side, and FIG. 3 is a side view of the shift detector 7, ie, a view seen from the rotation axis direction of the scanner.

  The shift detector 7 includes a scintillator 11 that converts X-rays into light, a photoelectric conversion element 12 that receives light emitted from the scintillator 11 and outputs an electrical signal, and a substrate 13 on which the photoelectric conversion element 12 is mounted. The The amount of light emitted from the scintillator 11 changes in accordance with the X-ray dose incident on the scintillator 11, and the electric signal output from the photoelectric conversion element 12 changes in accordance with the amount of light received by the photoelectric conversion element 12, so that it enters the shift detector 7. An electrical signal corresponding to the X-ray dose is output.

  Since the scintillator 11 transmits visible light, it is desirable to block visible light incident on the scintillator 11 from the outside. When the visible light from the outside is not blocked, the incident visible light passes through the scintillator 11, reaches the photoelectric conversion element 12, and is output as a noise signal. As an example of the light shielding method, there is a method of applying to the surface of the scintillator 11 a black resin obtained by mixing an epoxy resin with a material having a high light absorption rate such as graphite powder. As another method, there is a method in which a white resin obtained by mixing an epoxy resin with a material having high light reflectance such as titanium oxide powder is applied to the surface of the scintillator 11. When white resin is used, light generated in the scintillator 11 can be efficiently guided to the photoelectric element 12. After applying the white resin, a black resin may be further applied.

  The scintillator 11 is separated into sensitive regions 14 and 15 by a separation band 16, and the photoelectric conversion element 12 is also separated so as to correspond to the sensitive regions 14 and 15. The shift detector 7 having such a structure is irradiated with X-rays 4 which are a part of X-rays having a predetermined thickness W in the slice direction shaped by the collimator 6. The positional relationship between the X-ray 4 and the shift detector 7 shown in FIG. 2 is a state in which the position of the X-ray generation point is not shifted.

  FIG. 4 shows a system block diagram for compensating for the variation in the position of the X-ray generation point based on the output of the shift detector 7. The shift detector 7 outputs the outputs of the photoelectric conversion elements 12 corresponding to the sensitive regions 14 and 15, respectively, as detection signals S1 and S2. Detection signals S1 and S2 of the shift detector 7 are input to the position control device 9, and the position control device 9 controls the position adjustment device 17 based on S1 and S2.

  In the case of the positional relationship of the X-ray 4 shift detector 7 shown in FIG. 2, the X-rays are equally incident on the sensitive regions 14 and 15, and the detection signals S1 and S2 corresponding to the sensitive regions 14 and 15 are equal. Value. When the position of the X-ray generation point is shifted in the direction of the dotted line a or b shown in FIG. 2, a difference occurs between the detection signals S1 and S2. For example, when shifting in the direction of the dotted line a in FIG. 2, S1 <S2 and in the direction of the dotted line b, S1> S2. In other words, the difference (S1-S2) between the detection signals S1 and S2 becomes the shift amount, and the position controller 9 calculates the difference (S1-S2) to detect the position adjustment device 16 such as the X-ray source 1 or the support mechanism 5. If the control is performed so that the difference value between the signals S1 and S2 becomes zero, the position of the X-ray generation point can be compensated.

  Here, the characteristics of the structure of the shift detector 7 of the present invention will be described in comparison with the conventional shift detector 7 ′ shown in FIG. The mutual positional relationship between the photoelectric conversion element 12 and the substrate 13 is not different between the conventional shift detector 7 ′ and the shift detector 7 of the present invention, and on mounting such as damage prevention of the photoelectric conversion element 12 and securing of wiring space. Due to restrictions, the photoelectric conversion element 12 is arranged without protruding from the substrate 13.

  A scintillator 11 is further disposed on the photoelectric conversion element 12, but the scintillator 11 is disposed without protruding from the photoelectric conversion element 12 in the conventional shift detector 7 ′. On the other hand, in the shift detector 7 of the present invention, the scintillator 11 protrudes from the substrate 13 toward the rotation center side of the scanner. By adopting such a structure, the angle β can be made narrower than the structure of the conventional shift detector 7 ′. The reason will be described below.

  Since the detection signal of the shift detector 7 is proportional to the area of the X-ray 4 incident on the scintillator 11, in order to make the detection signal of the shift detector 7 more than a value determined by the required detection accuracy, the scintillator 11 The area of the incident X-ray 4 needs to be a predetermined value or more. In the structure in which the scintillator 11 protrudes from the substrate 13, the angle β is determined only by the area of the X-ray 4 incident on the scintillator 11 (see FIG. 3). In contrast, in the structure of the conventional shift detector 7 ′, in addition to the area of the X-rays incident on the scintillator 11 (corresponding to the angle β1 in FIG. 5), the X-rays irradiated on the substrate 13 (in FIG. 5) The angle β (= β1 + β2) is determined including the area of the angle β2). That is, in the structure in which the scintillator 11 protrudes, the area of the X-rays irradiated to the shift detector 7 is reduced by an amount corresponding to the angle β2, so that the angle β can be narrowed.

  Further, in the present embodiment, the X-ray 4 in the range of the angle β is irradiated only to the scintillator 11 and is not irradiated to the substrate as in the prior art. Therefore, in the present embodiment, scattered radiation due to X-rays irradiated to the substrate is not generated, so that it is possible to reduce the invalid exposure of the subject.

Next, a second embodiment will be described with reference to the drawings. FIG. 6 shows the shift detector 7 of the second embodiment. The difference from the first embodiment is the structure of the shift detector 7. In the structure of FIG. 3, the scintillator 11, the photoelectric element 12, and the substrate 13 are arranged in this order from the X-ray source 1 side. In the present embodiment, the substrate 13, the photoelectric element 12, and the scintillator 11 are arranged in this order. In such a configuration, the position where the X-ray 4 is detected is farther from the X-ray source 1 than in the case of FIG. Since the irradiation range of X-rays increases as the distance from the X-ray source 1 increases, detection of X-rays at a location distant from the X-ray source 1 makes it possible to detect smaller fluctuations in the X-ray position. That is, the detection accuracy of the shift detector 7 can be improved by detecting the position of the X-ray at a location farther from the X-ray source 1.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to these, Various deformation | transformation can be implemented.

The figure for demonstrating the whole structure of this invention. The top view of the shift detector 7 of 1st embodiment. The side view of the shift detector 7 of 1st embodiment. The block diagram of the shift compensation system of the X-ray generation point. The side view of the conventional shift detector 7 '. The side view of the shift detector 7 of 2nd embodiment.

Explanation of symbols

  1 X-ray source, 2 X-ray detector, 3 X-ray in the range of angle α, 4 X-ray in the range of angle β, 5 Support mechanism, 6 Collimator, 7 Shift detector, 8 Collimator for shift detector, 9 Position control Device, 10 subject, 11 scintillator, 12 photoelectric conversion element, 13 substrate, sensitive region corresponding to 14 S1, sensitive region corresponding to 15 S2, 16 separation band, 17 position adjustment device.

Claims (1)

Equipped with an X-ray source for irradiating a subject with X-rays, an X-ray detector arranged to face the X-ray source and detecting X-rays transmitted through the subject, and the X-ray source and the X-ray detector A scanner that rotates around the subject, an image reconstruction device that reconstructs a tomographic image of the subject based on the transmitted X-ray dose detected by the X-ray detector, and a tomogram that is reconstructed by the image reconstruction device An image display device that displays an image; a scintillator that converts X-rays into light; a photoelectric conversion element that receives light emitted from the scintillator to generate an electrical signal; and a substrate on which the photoelectric conversion element is mounted. A shift detector that detects the shift amount of the position of the X-ray incident on the X-ray detector, and an X that is incident on the X-ray detector after shifting the position of the X-ray source based on the detection value of the shift detector X-ray shift device for shifting the position of the line In X-ray CT apparatus,
The scintillator has a protrusion that protrudes toward the rotation center of the scanner from the end of the substrate, and the protrusion is located in the X-ray irradiation field .
The X-ray CT apparatus , wherein the shift detector is arranged in the order of a substrate, a photoelectric conversion element, and a scintillator from the X-ray source side .

Images