JP5107568B2 - X-ray CT system - Google Patents

Description

  The present invention relates to an X-ray CT (Computed Tomography) apparatus, and in particular, based on projection data (data) obtained by scanning an imaging position of an object set through scout imaging with X-rays. The present invention relates to an X-ray CT apparatus having an imaging unit that performs image reconstruction and a control unit that controls the imaging unit.

  In the X-ray CT apparatus, a subject is scanned with X-rays, and an image is reconstructed based on the obtained projection data. Prior to scanning, scout imaging of a subject is performed, and a position or range to be scanned is set using a scout image (see, for example, Patent Document 1).

The obtained projection data is subjected to beam hardening correction as part of the pre-processing so as not to cause a CT value shift due to X-ray quality hardening. Beam hardening correction is performed using a predetermined correction coefficient. The correction coefficient is obtained through a scanning experiment on a phantom (see, for example, Patent Document 2).
JP 2006-110183 A JP 2004-313524 A

  In actual subjects, the degree of X-ray quality hardening varies depending on the imaging region, X-ray irradiation direction, etc., so it is difficult to perform perfect beam hardening correction using the correction coefficient obtained from the phantom. Thus, streak artifacts and the like are likely to occur in the reconstructed image.

  Accordingly, an object of the present invention is to realize an X-ray CT apparatus that can easily perform beam hardening correction.

  An invention for solving the problem includes an imaging unit that performs image reconstruction based on projection data obtained by scanning an imaging position of a subject set through scout imaging with X-rays, and a control unit that controls the imaging unit. The X-ray CT apparatus has an X-ray beam line during a scan according to the magnitude of the attenuation amount, wherein the control unit obtains an attenuation amount of transmitted X-rays at a planned imaging location from the result of the scout imaging. An X-ray CT apparatus characterized by changing the quality.

  The X-ray beam quality is changed by increasing or decreasing the tube voltage of the X-ray tube. The amount of attenuation of the transmitted X-rays is the amount of radiation hardening. The X-ray beam quality is changed according to the view angle.

  According to the present invention, an X-ray CT apparatus controls an imaging unit that performs image reconstruction based on projection data obtained by scanning an imaging location of a subject set through scout imaging with X-rays. A control unit, which determines the amount of transmitted X-ray attenuation at the planned imaging location from the result of the scout imaging, and changes the X-ray beam quality during scanning according to the amount of attenuation. Therefore, an X-ray CT apparatus with easy beam hardening correction can be realized.

  The best mode for carrying out the invention will be described below with reference to the drawings. Note that the present invention is not limited to the best mode for carrying out the invention. FIG. 1 shows a schematic configuration of an X-ray CT apparatus. This apparatus is an example of the best mode for carrying out the present invention. An example of the best mode for carrying out the invention related to the X-ray CT apparatus is shown by the configuration of the apparatus.

  The apparatus has a gantry 100, a table 200, and an operator console 300. The gantry 100 scans the subject 10 carried by the table 200 with the X-ray irradiation / detection device 110, collects projection data of a plurality of views, and inputs it to the operator console 300.

  The operator console 300 performs image reconstruction based on the projection data input from the gantry 100 and displays the reconstructed image on a display 302. Image reconstruction is performed by a dedicated computer in the operator 300. The computer for image reconstruction, the gantry 100, and the table 200 are examples of the photographing unit in the present invention.

  The operator console 300 also controls the operation of the gantry 100 and the table 200. Control is performed by a dedicated computer in the operator 300. This computer is an example of a control unit in the present invention. The control unit also controls image reconstruction.

  Under the control of the operator console 300, the gantry 100 scans under a predetermined scanning condition, and the table 200 positions the subject 10 so that a predetermined part is scanned. Positioning is performed by adjusting the height of the top plate 202 and the horizontal movement distance of the cradle 204 on the top plate by a built-in position adjustment mechanism.

 A helical scan can be performed by continuously performing a plurality of scans while continuously moving the cradle 204. A cluster scan can be performed by scanning each cradle 204 while intermittently moving the cradle 204. By continuously moving the cradle 204 with the rotation of the X-ray irradiation / detection device 110 stopped, a scout scan can be performed.

  An axial scan can be performed by scanning with the cradle 204 stopped. A cine scan can be performed by continuously performing an axial scan a plurality of times.

  The height adjustment of the top plate 202 is performed by swinging the support column 206 around the attachment portion to the base 208. The top plate 202 is displaced in the vertical direction and the horizontal direction by the swing of the column 206. The cradle 204 moves in the horizontal direction on the top plate 202 to cancel the horizontal displacement of the top plate 202. Depending on the scan conditions, the scan is performed with the gantry 100 tilted. The gantry 100 is tilted by a built-in tilt mechanism.

  As shown in FIG. 2, the table 200 may be of a type in which the top plate 202 moves up and down vertically with respect to the base 208. The top plate 202 is moved up and down by a built-in lifting mechanism. In this table 200, the horizontal movement of the top plate 202 accompanying the raising and lowering does not occur.

  FIG. 3 schematically shows the configuration of the X-ray irradiation / detection device 110. The X-ray irradiation / detection device 110 detects an X-ray 134 emitted from the focal point 132 of the X-ray tube 130 with an X-ray detector 150.

  The X-ray 134 is shaped by a collimator (not shown) and becomes a cone beam or fan beam X-ray. The X-ray detector 150 has an X-ray incident surface 152 that expands two-dimensionally corresponding to the spread of X-rays. The X-ray incident surface 152 is curved so as to constitute a part of a cylinder. The central axis of the cylinder passes through the focal point 132.

  The X-ray irradiation / detection device 110 rotates around a central axis passing through an imaging center, that is, an isocenter O. The central axis is parallel to the central axis of the partial cylinder formed by the X-ray detector 150.

  The direction of the center axis of rotation is the z direction, the direction connecting the isocenter O and the focal point 132 is the y direction, and the direction perpendicular to the z direction and the y direction is the x direction. These x, y, and z axes are three axes in a rotating coordinate system with the z axis as the central axis.

  FIG. 4 schematically shows a plan view of the X-ray incident surface 152 of the X-ray detector 150. The X-ray incident surface 152 has detection cells 154 arranged two-dimensionally in the x direction and the z direction. That is, the X-ray incident surface 152 is a two-dimensional array of detection cells 154. In the case of using fan beam X-rays, the X-ray incident surface 152 may be a one-dimensional array of detection cells 154.

  Each detection cell 154 constitutes a detection channel. Thereby, the X-ray detector 150 becomes a multi-channel X-ray detector. The detection cell 154 is configured by, for example, a combination of a scintillator and a photodiode.

  FIG. 5 shows a block diagram of the tube voltage supply system of the X-ray tube 130. As shown in FIG. 5, the X-ray tube 130 is supplied with a tube voltage from a high voltage generator 140. The high voltage generator 140 includes a high voltage inverter & insulation unit 142 and a tube voltage control unit 144.

 The high voltage inverter & insulation unit 142 applies a high voltage to the X-ray tube 130 under the control of the tube voltage control unit 144. Tube voltage control by the tube voltage control unit 144 is performed by feedback control. The tube voltage control unit 144 is constituted by, for example, firmware.

 Tube voltage control by the tube voltage control unit 144 is performed based on control information given from the operator console 300. Control information is determined by a system software scan plan.

  FIG. 6 shows a flowchart of the operation of the present apparatus. This operation is performed under the control of the operator console 300. As shown in FIG. 6, in step 601 a scout scan is performed. Scout scanning is performed by continuously moving the cradle 204 while irradiating X-rays while the rotation of the X-ray irradiation / detection device 110 is stopped.

 The X-ray irradiation direction is a minor axis direction or a major axis direction when the cross-sectional shape of the subject 10 is regarded as an ellipse. The minor axis direction corresponds to a 0 degree direction or a 180 degree direction of the rotation angle of the X-ray irradiation / detection device 110. The major axis direction corresponds to the 90 degree direction or the 270 degree direction of the rotation angle of the X-ray irradiation / detection device 110. The scout scan may be performed in the minor axis direction and the major axis direction, respectively.

 In step 602, localization is performed. Localization is performed by the operator through the operator console 300. Thereby, for example, the imaging range L of the helical scan as shown in FIG. 7 is set. The imaging range L is a range from positions A to B on the body axis of the subject 10.

 In step 603, the amount of X-ray attenuation at the planned imaging position is obtained. The amount of X-ray attenuation is obtained from projection data obtained by a scout scan. As a result, as shown in FIG. 8, a profile of the X-ray attenuation along the body axis is obtained. The amount of X-ray attenuation is relatively large in the shoulders and buttocks and relatively small in the chest and abdomen. Such a difference in the amount of X-ray attenuation is caused by a difference in the proportion of bone in the X-ray transmission length, and the greater the proportion of bone, the greater the attenuation.

 Where the X-ray attenuation is large, the degree of radiation hardening is large, and where the X-ray attenuation is small, the degree of radiation hardening is small. For this reason, the magnitude of the X-ray attenuation corresponds to the magnitude of the quality hardening. Therefore, the degree of radiation quality hardening can be predicted from the profile of the amount of X-ray attenuation.

 In step 604, a variable energy scan is performed. The variable energy scan is performed by changing the tube voltage applied to the X-ray tube 130 during the scan.

 FIG. 9 shows a tube voltage change pattern. As shown in FIG. 9, the tube voltage is changed into two stages of High kV and Low kV during the helical scan. High kV is a relatively high voltage, and Low kV is a relatively low voltage. Generally, High kV is about 140 kV, and Low kV is about 80 kV. At high kV, the X-ray beam quality is hard, and at low kV, the X-ray beam quality is soft.

 A high voltage High kV is applied when scanning the shoulder and buttocks, and a low voltage Low kV is applied when scanning the chest and abdomen. The shoulder portion and the heel portion have a high degree of radiation hardening, and high-energy X-rays are used for scanning this portion. The chest and abdomen have a low degree of radiation hardening, and X-rays with low energy are used for scanning this part.

 The shoulder and buttocks have a high degree of radiation hardening, but by making the X-ray beam quality harder at this part, the amount of X-ray transmission can be increased. This degree can be relaxed to the same extent as radiation hardening in the chest and abdomen.

 As a result, the degree of radiation quality hardening is equal even in places where there are many bones and where there are few bones, so that beam hardening correction is facilitated, and the reconstructed image has a high image quality that does not include streak artifacts.

 The tube voltage may be changed during the axial scan. That is, for example, as shown in FIG. 10, when the axial scan is performed at the slice position a including the shoulder, the tube voltage is changed in a pattern as shown in FIG.

 As shown in FIG. 11, the tube voltage change pattern is such that, during an axial scan of 360 degrees, the view angle is 90 degrees and 270 degrees and high kV in the vicinity thereof, the view angle is 0, 180, 360 degrees and It is set to Low kV in the vicinity.

 The view angles of 90 degrees and 270 degrees are major axis directions when the cross-sectional shape of the subject 10 is regarded as an ellipse, and are directions in which the X-ray attenuation is large. The view angles of 0, 180, and 360 degrees are directions of the minor axis when the cross-sectional shape of the subject 10 is regarded as an ellipse, and are directions in which the amount of X-ray attenuation is small.

 In the direction in which the amount of X-ray attenuation is large, the tube voltage is set to High kV, the X-ray beam quality is hardened, the degree of quality hardening is relaxed, and the quality of the view is comparable to that at 0, 180, 360 degrees. This facilitates beam hardening correction, and a high-quality reconstructed image that does not include streak artifacts can be obtained. Such tube voltage control may also be performed during helical scanning.

It is a figure which shows the structure of the X-ray CT apparatus of an example of the best form for implementing this invention. It is a figure which shows the structure of the X-ray CT apparatus of an example of the best form for implementing this invention. It is a figure which shows the structure of a X-ray irradiation / detection apparatus. It is a figure which shows the structure of the X-ray entrance plane of an X-ray detector. It is a figure block diagram which shows a tube voltage supply system. It is a flowchart which shows operation | movement of the X-ray CT apparatus of an example of the best form for implementing this invention. It is a figure which shows an example of localization. It is a figure which shows an example of an X-ray attenuation amount profile. It is a figure which shows an example of a tube voltage change pattern. It is a figure which shows an example of localization. It is a figure which shows an example of a tube voltage change pattern.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10: Subject 100: Gantry 110: X-ray irradiation / detection apparatus 130: X-ray tube 132: Focus 134: X-ray 150: X-ray detector 152: X-ray incident surface 154: Detection cell 200: Table 202: Top plate 204: Cradle 206: Supporting column 208: Base 300: Operator console 302: Display 140: High voltage generator 142: High voltage inverter & insulation unit 144: Tube voltage control unit

Claims (4)

An X-ray CT apparatus having an imaging unit that performs image reconstruction on the basis of projection data obtained by scanning an imaging location of a subject set through scout imaging and a control unit that controls the imaging unit,
The control unit increases the tube voltage of the X-ray tube when the amount of attenuation of transmitted X-rays at the imaging scheduled location obtained from the result of the scout imaging is large during scanning, and when the amount of attenuation is small An X-ray CT apparatus for reducing the tube voltage . The X-ray CT apparatus according to claim 1, wherein the control unit changes the tube voltage according to a position of the subject in the body axis direction . The X-ray CT apparatus according to claim 1 , wherein the control unit changes the tube voltage according to a view angle . Wherein the control unit, the tube voltage, X-rays CT apparatus according to any one of claims 1 to 3 to switch to one of the relatively high voltage and a relatively low voltage.

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