JP5606667B2 - X-ray CT system - Google Patents

Description

  The present invention relates to an X-ray CT (Computed Tomography) apparatus, and more particularly to an X-ray CT apparatus having an X-ray control unit that controls imaging conditions and an image reconstruction unit.

  The X-ray CT apparatus scans a subject with X-rays and reconstructs an image based on the obtained projection data. The X-ray CT apparatus of Patent Document 1 performs scout imaging of a subject prior to scanning, and sets the position and range to be scanned by referring to the scout image.

Projection data obtained by scanning is subjected to beam hardening correction as part of the pre-processing so that CT value shifts and artifacts due to X-ray quality hardening do not occur. Yes. According to Patent Document 2, beam hardening correction is performed using a predetermined correction coefficient, and the correction coefficient is obtained by an experiment using a phantom or the like.
JP 2006-110183 A JP 2004-313524 A

  However, in an actual subject, the degree of X-ray quality hardening varies depending on the imaging region, X-ray irradiation direction, and the like, and therefore high-precision beam hardening correction is performed using the correction coefficient obtained from the phantom. This may be difficult, and streak artifacts may occur in the reconstructed image.

  There is also a technique for making the X-ray tube current variable according to the body shape of the subject. However, depending on the body shape of the subject, the X-ray tube current may partially exceed the upper limit of the usable X-ray tube current. If an X-ray tube current that does not exceed the upper limit is used, the X-ray dose is small and a clear reconstructed image cannot be obtained.

  Accordingly, an object of the present invention is to provide a uniform image quality between a plurality of tomographic images in the imaging region even when there is a portion where the degree of radiation hardening or the X-ray dose is insufficient in the imaging region of the subject. It is to provide an X-ray CT apparatus capable of obtaining a tomographic image.

  An X-ray CT apparatus according to a first aspect is an X-ray CT apparatus that performs image reconstruction based on X-ray projection data obtained by scanning an imaging region of a subject with X-rays from an X-ray generation unit. An X-ray control unit that changes an X-ray beam quality of the X-ray irradiated to the subject during a scan according to a position of the subject in the body axis direction in the imaging region of the subject; And an image reconstructing unit for reconstructing a plurality of tomographic images corresponding to the uniform X-ray beam quality in the imaging region based on the X-ray projection data based on the X-ray beam quality.

  The X-ray controller of the X-ray CT apparatus according to the second aspect changes the X-ray beam quality by increasing or decreasing the X-ray tube voltage of the X-ray generator.

  The X-ray control unit of the X-ray CT apparatus according to the third aspect changes the X-ray beam quality according to the position of the subject in the body axis direction based on the subject information obtained by scout imaging of the subject. .

  In the third aspect, the X-ray control unit of the X-ray CT apparatus according to the fourth aspect is configured so that the X-ray absorption amount specified by the scout imaging of the subject is within the imaging region with respect to a position larger than a predetermined reference. X-ray beam quality with higher energy than other positions.

  In the third aspect, the X-ray controller of the X-ray CT apparatus according to the fifth aspect has an X-ray tube current condition calculated based on the geometric feature of the subject specified by scout imaging of the subject. The X-ray beam quality with higher energy than the other positions in the imaging region is set as the X-ray tube current condition below the predetermined reference with respect to the position larger than the predetermined reference.

  The X-ray control unit of the X-ray CT apparatus according to the sixth aspect provides an X-ray having a first energy spectrum from the X-ray generation unit and an X-ray having a second energy spectrum different from the first energy spectrum in scout imaging. The position of each substance contained in the subject is acquired by irradiating the subject at least for each view, and the image reconstruction unit obtains X-rays based on the X-ray beam quality changed by the X-ray control unit. Projection data or tomographic image data obtained by image reconstruction using the X-ray projection data is converted into uniform X-ray beam quality equivalent data by a conversion coefficient using the X-ray absorption coefficient of the substance. A plurality of tomographic images corresponding to uniform X-ray beam quality are obtained in the imaging region.

  The X-ray control unit of the X-ray CT apparatus according to the seventh aspect has an X-ray having the first energy spectrum and a second energy spectrum different from the first energy spectrum at a position where the X-ray beam quality is changed. The X-ray is changed at least for each view, and the image reconstruction unit performs first X-ray having a first energy spectrum on the X-ray projection data or tomographic image data at the position where the X-ray beam quality is changed. X-ray projection data and X-ray second X-ray projection data of an X-ray having a second energy spectrum, or X for the substance of the subject specified based on the first X-ray projection data and the second X-ray projection data The data is converted into data corresponding to a uniform X-ray beam quality by a conversion coefficient using a linear absorption coefficient, and a plurality of tomographic images corresponding to a uniform X-ray beam quality in the imaging region are obtained.

An X-ray CT apparatus according to an eighth aspect according to the first to seventh aspects includes an X-ray beam detector that detects the quality of the X-ray beam irradiated from the X-ray generation unit, and associates with X-ray projection data. The X-ray beam quality detected in this way is stored.
The X-ray beam quality of the X-ray CT apparatus according to the ninth aspect is an X-ray tube voltage.
In an X-ray CT apparatus according to a tenth aspect, an X-ray tube voltage used for X-rays irradiated from the X-ray generation unit is stored in association with the X-ray projection data.

  The X-ray CT apparatus of the present invention is uniform between a plurality of tomographic images in the imaging region even when there is a portion where the degree of radiation hardening or the X-ray dose is insufficient in the imaging region of the subject. An X-ray CT apparatus capable of obtaining a high-quality tomographic image can be realized.

(Embodiment 1)
First, the overall configuration of the X-ray CT apparatus 100 used in the present embodiment will be described.
<Overall configuration of X-ray CT apparatus 100>
FIG. 1 is a configuration block diagram of an X-ray CT apparatus 100 of the present embodiment. The X-ray CT apparatus 100 includes an operation console 1, an imaging table 10, and a scanning gantry 20.

  The operation console 1 includes an input device 2 such as a keyboard or a mouse that receives input from the operator, a central processing unit 3 that executes preprocessing, image reconstruction processing, postprocessing, and the like, and X-ray detection collected by the scanning gantry 20 And a data collecting unit 5 for collecting the vessel data. Further, the operation console 1 includes a monitor 6 that displays a tomographic image reconstructed from projection data obtained by preprocessing X-ray detector data, a program, X-ray detector data, projection data, and X-ray tomography. And a storage device 7 for storing an image. The photographing condition is input from the input device 2 and stored in the storage device 7. The imaging table 10 includes a cradle 12 on which the subject HB is placed and taken in and out of the opening of the scanning gantry 20. The cradle 12 is moved up and down and linearly moved in the Z direction by a motor built in the imaging table 10.

  The scanning gantry 20 includes an X-ray tube 21, an X-ray control unit 22, a multi-row X-ray detector 24, and a data acquisition device 25 (DAS: Data Acquisition System). A collimator 23 and a beam forming X-ray filter 28 are disposed between the X-ray tube 21 and the subject HB. Further, the scanning gantry 20 includes a rotation control unit 26 that performs rotation control of the rotation unit 15 including the X-ray tube 21 that rotates around the body axis of the subject HB, and a control signal and the like to the operation console 1 and the imaging table 10. A gantry control unit 29 is provided. The X-ray control unit 22 controls the X-ray tube voltage and the X-ray tube current mA to the X-ray tube 21. The X-ray control unit 22 can perform dual energy imaging while switching between a low X-ray tube voltage, for example, an X-ray tube voltage of 80 kV, and a high X-ray tube voltage, for example, an X-ray tube voltage of 140 kV, every one or several views. . Further, an X-ray tube voltage detector 31 for detecting a change in actual X-ray tube voltage is disposed in the vicinity of the X-ray tube 21. Instead of the X-ray tube voltage detector 31, an X-ray tube voltage detector 31 'arranged near the multi-row X-ray detector 24 may be prepared.

  The beam forming X-ray filter 28 is a filter that increases X-rays toward the rotation center, which is the imaging center, and decreases the X-ray dose in the peripheral part. For this reason, exposure of the body surface of the subject HB close to a circle or an ellipse can be reduced.

The central processing unit 3 includes a preprocessing unit 33 and an image reconstruction unit 34.
The preprocessing unit 33 performs preprocessing such as correction of sensitivity between channels, offset correction, logarithmic conversion, and X-ray dose correction on the projection data collected by the data collection device 25. Also, beam hardening processing is performed.

  The image reconstruction unit 34 receives the projection data preprocessed by the preprocessing unit 33 and reconstructs an image based on the projection data. The projection data is subjected to Fast Fourier Transform (FFT) for transforming into the frequency domain, and the reconstruction function Kernel (j) is superimposed on the Fourier transform and inverse Fourier transform is performed. Then, the image reconstruction unit 34 performs a three-dimensional backprojection process on the projection data obtained by superimposing the reconstruction function Kernel (j), and obtains a tomographic image for each body axis direction (Z-axis direction) of the subject HB. (Xy plane) is obtained. The image reconstruction unit 34 stores this tomographic image in the storage device 7.

  Further, the image reconstruction unit 34 uses, for example, projection data with an X-ray tube voltage of 80 kV and projection data with an X-ray tube voltage of 140 kV to obtain a two-dimensional distribution tomography of X-ray tube voltage-dependent information related to the distribution of a predetermined substance (atom). Reconstruct the image. As a tomographic image of dual energy imaging, a water equivalent image, a fat equivalent image, a contrast agent equivalent image, a bone equivalent image, and the like can be obtained. Hereinafter, imaging using a plurality of different X-ray tube voltages is referred to as dual energy imaging.

  Further, the image reconstruction unit 34 can perform reprojection processing by rotating the tomographic image in the θ direction based on various equivalent images, or can perform reprojection processing by shifting the tomographic image.

Next, the operation of the X-ray CT apparatus according to this embodiment will be described.
<Operation Flowchart of X-ray CT Apparatus 100>
FIG. 2 is a flowchart showing an outline of the operation of the X-ray CT apparatus 100 of the present embodiment.

In step D1, the operator places the subject on the cradle 12 of the imaging table 10 and performs alignment. Here, the subject HB placed on the cradle 12 aligns the slice light center position of the scanning gantry 20 with the reference point of each part.
In step D2, an X-ray tube voltage of 140 kV and an X-ray tube voltage of 80 kV at a low dose (low X-ray tube current) are applied to each view or multiple views as shown in FIGS. 3 (a) and 3 (b). Dual energy imaging of helical scout scan, which collects X-ray data while switching to.

  In FIG. 3A, X-ray data acquisition with an X-ray tube voltage of 80 kV is performed in an odd-numbered view, and X-ray data acquisition with an X-ray tube voltage of 140 kV is performed in an even-numbered view. In FIG. 3B, X-ray data collection with an X-ray tube voltage of 80 kV and X-ray data collection with an X-ray tube voltage of 140 kV are alternately repeated every two or three consecutive views every three views. Is going. The X-ray tube voltage value of the X-ray tube voltage controlled by the X-ray controller 22 is stored at the time of X-ray acquisition, or the actual X-ray tube voltage is the X-ray tube voltage. The X-ray tube voltage detected by the measurement detector 31 and detected by the X-ray tube voltage measurement detector 31 at the time of X-ray data collection is simultaneously stored.

  In general, when N-view X-ray data acquisition is performed with one rotation, X-ray projection data of an odd-view X-ray tube voltage of 80 kV is N / 2 view, and X-ray projection data of an even-view X-ray tube voltage of 140 kV is N. / 2 views, and the number of views is reduced to 1/2 of each X-ray projection data. In the X-ray projection data of N / 2 view, there is a possibility that aliasing artifacts appear in the peripheral portion in a large field of view. For this reason, the image reconstruction unit 34 interpolates X-ray projection data by interpolation processing in the view direction or weighted addition processing, and sets the X-ray projection data of each N / 2 view to N views for aliasing. Prevent artifacts. There is also a method of switching the X-ray tube voltage every other view by doubling the number of views per rotation.

  For example, in a 1-second scan with 1000 views per rotation, the view interval is in units of 1 millisecond, and in a 0.5-second scan with 1000 views in 1 rotation, the view interval is in units of 0.5 milliseconds. , Beat can be suppressed.

  According to the helical scout scan using the dual energy imaging scan method of the present embodiment, a scout image of 0 degree direction and 90 degree direction, or an arbitrary angle can be obtained by one low exposure helical scout scan. it can. A scout image of an arbitrary angle is obtained by obtaining a continuous tomographic image in the Z direction once by a helical scout scan, and performing a parallel beam reprojection process in the direction in which the scout image is to be obtained, thereby generating a scout image in a desired direction without geometric distortion. Can get. The imaging time is a helical scan using a high-speed helical pitch with a high speed using all rows of the X-ray detector width in the Z direction, so it can be performed in a shorter time than conventional scout imaging, and in multiple directions. Scout images can be reconstructed in a short time.

  Returning to FIG. 2 again, in step D3, only X-ray projection data having an X-ray tube voltage of 140 kV is extracted, X-ray projection data having an X-ray tube voltage of 140 kV is obtained, and X-ray projection data having an X-ray tube voltage of 80 kV is obtained. Are extracted, and X-ray projection data with an X-ray tube voltage of 80 kV is obtained. A method for extracting X-ray projection data will be described later.

  In step D4, the image reconstruction unit 34 reconstructs a tomographic image having an X-ray tube voltage of 140 kV from X-ray projection data having an X-ray tube voltage of 140 kV, and X-ray tube voltage from the X-ray projection data having an X-ray tube voltage of 80 kV. An 80 kV tomographic image is reconstructed.

  In step D5, the image reconstruction unit 34 obtains a dual energy ratio tomogram. The dual energy ratio tomographic image is obtained by obtaining a ratio of a tomographic image based on the X-ray tube voltage 80 kV and a tomographic image based on the X-ray tube voltage 140 kV for each pixel. As the dual energy ratio tomogram, a three-dimensional dual energy ratio tomogram continuous in the Z direction may be obtained. The position information of each substance contained in the subject can be acquired using this dual energy specific tomographic image. That is, for example, as shown in FIG. 4, on the graph in which the pixel value of the tomographic image based on the X-ray tube voltage 80 kV and the pixel value of the tomographic image based on the X-ray tube voltage 140 kV are plotted, By setting the slope range, the pixels of the points included in each range can be associated with each substance.

  In step D6, the scout image is reconstructed and displayed. In the helical scout scan, a re-projection process RP is performed on the three-dimensional image of the X-ray tube voltage 80 kV and / or the three-dimensional image of the X-ray tube voltage 140 kV continuous in the Z direction in the 0 degree direction or 90 degree direction, and the scout image is obtained. Image reconstruction can be performed. A scout image composed of a tomographic image with an X-ray tube voltage of 80 kV and / or a scout image composed of a tomographic image with an X-ray tube voltage of 140 kV, and a dual energy ratio tomographic image as required can be displayed.

In step D7, shooting conditions for the main scan are set.
In the present embodiment, the X-ray control unit 22 can select and use a plurality of scan patterns such as a conventional scan, a helical scan, a cine scan or a helical shuttle scan, and a variable pitch helical scan. The conventional scan is a scan method in which the X-ray tube 21 and the multi-row X-ray detector 24 are rotated to acquire X-ray projection data every time the cradle 12 is moved at a predetermined interval in the z-axis direction. The helical scan is an imaging method for collecting X-ray projection data by moving the cradle 12 at a constant speed while the gantry rotating unit 15 rotates. The helical shuttle scan is a scan method in which the cradle 12 is accelerated and decelerated while rotating the gantry rotating unit 15 as in the helical scan, and the X-ray projection data is collected by reciprocating in the positive or negative direction of the z axis. It is. In that case, X-ray projection data during acceleration and deceleration may be collected. The variable pitch helical scan is a method of collecting X-ray projection data while changing the helical pitch by a helical scan in only one direction.
In addition, the X-ray controller 22 sets the X-ray tube voltage and the X-ray tube current for each Z-direction coordinate position and view direction. For example, the X-ray control unit 22 performs an imaging plan by displaying a scout image obtained by reprojecting a tomographic image continuous in the Z direction by a helical scout scan.

  Further, the operator can set the imaging conditions for each Z-direction coordinate position of the main scan by looking at the size of the subject at each Z-direction coordinate position. For example, as shown in FIG. 8A, there is a bone in the range of [z1, z2] including the shoulder, and X-ray absorption is large. For this reason, as shown in FIG. 8B, in the Z direction range [z0, z1], the X-ray tube voltage is 120 kV, the X-ray tube current mA1, and in the Z direction range [z2, z3], the X-ray tube voltage is 120 kV. Thus, in the X-ray tube current mA2 and the Z direction range [z3, z4], the operator can set the X-ray tube current mA3 at the X-ray tube voltage of 120 kV. The X-ray tube current is the maximum X-ray tube current mAMAX. The X-ray tube voltage shown in FIG. 8B is an X-ray tube voltage used for irradiated X-rays or a value detected by the X-ray tube voltage measurement detector 31.

  Further, in the range of [z1, z2] including the shoulder shown in FIG. 5A, the transmission path in the X direction is long while the transmission path in the Y direction is short as shown in FIG. 6A. For this reason, as shown in FIG. 6B, in the view angle ranges [θ1, π−θ1] and [π + θ1, −θ1] viewed from the Y direction, the X-ray tube voltage is set to 140 kV, and the view angle is set. In the ranges [−θ1, θ1] and [π−θ1, π + θ1], the X-ray tube voltage can be set to 120 kV.

FIG. 7 is a screen on which the operator sets the X-ray tube voltage kV or the X-ray tube current mA in the view angle range.
When the operator presses the X-ray tube voltage value setting button using the input device 2, a screen for selecting the fixed tube voltage mode FixkV or the view tube voltage mode ViewkV shown in FIG. The Further, when the operator presses the X-ray tube current value setting button using the input device 2, a screen for selecting the fixed tube current mode FixmA or the view tube current mode ViewmA shown in FIG. Is done.

  When the operator selects the view tube voltage mode ViewkV shown in FIG. 7A, a screen for setting each view direction range and X-ray tube voltage value indicated by arrows is displayed, and the movable view range indicated by a broken line. By moving MV, it is possible to set each view direction range and X-ray tube voltage value. The X-ray control unit 22 emits X-rays with the set X-ray tube voltage value.

  Similarly, when the operator selects the view tube current mode ViewmA shown in FIG. 7B, a screen for setting each view direction range indicated by the arrow and the X-ray tube current value is displayed, and the movable indicated by the broken line is displayed. Each view direction range and X-ray tube current value can be set by moving the view range MV. The X-ray controller 22 emits X-rays with the set X-ray tube current value.

In step D8, the main scan is performed according to the imaging conditions set above.
Here, when collecting data by helical scanning, the X-ray tube 21 and the multi-row X-ray detector 24 are rotated around the subject HB, and the cradle 12 on the imaging table 10 is moved linearly. The data collection operation of the X-ray detector data is performed. Then, X-ray detector data D0 (view, j, i) (j = 1 to ROW, i = 1 to CH) represented by the view angle view, the detector row number j, and the channel number i is necessary. The position information Ztable (view) of the z-axis coordinate is added. As described above, in the helical scan, X-ray detector data collection within a constant speed range is performed. The z-axis coordinate position information may be added to the X-ray projection data (X-ray detector data), or may be used in association with the X-ray projection data as a separate file. The position information of the z-axis coordinates is used when reconstructing the three-dimensional image of the X-ray projection data during the helical shuttle scan.
In step D9, the image reconstruction unit 34 performs image reconstruction processing as follows.
First, the X-ray detector data D0 (view, j, i) is subjected to preprocessing such as correction of sensitivity between channels, offset correction, logarithmic conversion, X-ray dose correction, etc., and converted into projection data.
Next, the preprocessing unit 33 performs beam hardening correction. Here, beam hardening correction is performed on the preprocessed projection data D1 (view, j, i). At this time, since independent beam hardening correction can be performed for each j column of the detector, if the tube voltage of each gantry rotating unit 15 is different under imaging conditions, the X-ray energy characteristics of the detector for each column Can be corrected. The beam hardening correction can also be processed for the profile area of the subject HB, the ellipticity, and various X equivalent substances.

  Next, the image reconstruction unit 34 performs z filter convolution processing. Here, a z-filter convolution process for applying a filter in the z-axis direction (column direction) is performed on the projection data D11 (view, j, i) subjected to beam hardening correction. That is, after pre-processing at each view angle, for example, a filter with a column direction filter size of 5 columns is applied to the projection data D11 (view, j, i) subjected to beam hardening correction.

Next, the image reconstruction unit 34 performs reconstruction function superimposition processing. That is, the Fourier transform (Fourier Transform) for transforming the X-ray projection data into the frequency domain is performed, the reconstruction function is multiplied, and the inverse Fourier transform is performed.
Next, the image reconstruction unit 34 performs a three-dimensional backprojection process. Here, three-dimensional backprojection processing is performed on the projection data D3 (view, j, i) subjected to the reconstruction function superimposition processing to obtain backprojection data D3 (x, y, z). The image to be reconstructed is a plane perpendicular to the z axis. The following reconstruction area is assumed to be parallel to the xy plane.

Next, the image reconstruction unit 34 performs post-processing. Post-processing such as image filter superimposition and CT value conversion is performed on the backprojection data D3 (x, y, z) to obtain a tomographic image.
In step D10, X-ray tube voltage correction of the tomographic image is performed so that a tomographic image corresponding to a uniform X-ray tube voltage is obtained by a conversion coefficient using an X-ray absorption coefficient for the substance of the subject. For example, X-rays at 120 kV of a substance included in a tomographic image so that a tomographic image of a section using an X-ray tube voltage of 140 kV becomes a tomographic image corresponding to an X-ray tube voltage of 120 kV used in another section. The tomographic image is corrected using a conversion coefficient based on the ratio between the absorption coefficient and the X-ray absorption coefficient at 140 kV.
In step D11, a tomographic image is displayed on the monitor 6.
Here, the extraction method of the X-ray projection data of the helical scout scan using the dual energy imaging scan method in step D3 will be described in detail.
FIG. 8 is a diagram showing an X-ray projection data extraction method and interpolation method. The X-ray projection data is extracted by dividing the helical scout scan X-ray projection data into X-ray projection data with an X-ray tube voltage of 80 kV and X-ray projection data with an X-ray tube voltage of 140 kV as shown in FIG. To do. The X-ray projection data of the N / 2 view X-ray tube voltage of 80 kV makes up for missing view data using interpolation processing or weighted addition processing. Similarly, the X-ray projection data of the X / 2 tube voltage of 140 kV in the N / 2 view compensates for missing view data using interpolation processing or weighted addition processing.

The X-ray projection data extraction method and interpolation processing perform the following processing. For example, X-ray projection data with an X-ray tube voltage of 80 kV is D80 (view, row, ch), and X-ray projection data with an X-ray tube voltage of 140 kV is D140 (view, row, ch). As shown in FIG. 8, when the X-ray tube voltage 80 kV and the X-ray tube voltage 140 kV are switched every two views, the collected X-ray projection data D (view, row, ch) is the X-ray tube every two views. The X-ray projection data having a voltage of 80 kV and the X-ray projection data having an X-ray tube voltage of 140 kV can be separated as in the following (Equation 01) to (Equation 08). However, k is an integer.
. . . (Formula 01)
. . . (Formula 02)
. . . (Formula 03)
. . . (Formula 04)
. . . (Formula 05)
. . . (Formula 06)
. . . (Formula 07)
. . . (Formula 08)

As an example, the interpolation processing can also be performed as in (Formula 09) to (Formula 12) below.
. . . (Formula 09)
. . . (Formula 10)
. . . (Formula 11)
. . . (Formula 12)
Next, the dual energy imaging in steps D2 to D6 described above will be described in detail.
FIG. 9 shows a flowchart of the image reconstruction RC in the helical scout scan. The following is a detailed description thereof.

In step H1, a low-exposure helical scout scan is performed at a plurality of X-ray tube voltages.
Low exposure helical scout scans do the following to reduce exposure:
1. The tomographic image of 512 × 512 pixels is set to 256 × 256 pixels for the image reconstruction matrix. Alternatively, the 512 × 512 pixel tomographic image G1 (x, y) is obtained by performing the filtering process of the following (Equation 18) on the 512 × 512 pixel tomographic image G (x, y) and superimposing four pixels of data on one pixel. ).
. . . (Formula 13)
2. The slice thickness d and the image interval p of the tomographic image in the Z direction are overlapped as shown in the following (Equation 14).
. . . (Formula 14)
3. A low-frequency function is used as the reconstruction function.
4). As the image filter, a low frequency filter or a filter having a noise reduction effect is used.
5. The exposure per unit length in the Z direction is reduced by setting the helical pitch to 1 or more.

  In step H2, the image reconstruction unit 34 performs image reconstruction RC of a tomographic image with an X-ray tube voltage of 80 kV and an X-ray tube voltage of 140 kV in a helical scout scan. The image reconstruction unit 34 separates the X-ray projection data with the X-ray tube voltage 80 kV and the X-ray projection data with the X-ray tube voltage 140 kV by the above-described X-ray projection data extraction method and interpolation method. The image reconstruction unit 34 obtains a tomographic image of the X-ray tube voltage 80 kV and a tomographic image of the X-ray tube voltage 140 kV from the X-ray projection data of the X-ray tube voltage 80 kV and the X-ray projection data of the X-ray tube voltage 140 kV. In the helical scout scan, image reconstruction RC of a calcium weighted image or a contrast agent weighted image may be performed using a dual energy image reconstruction method.

In Step H3, a reprojection process in the Y-direction view angle direction is performed on the tomographic images continuous in the Z direction by the reprojection process. That is, image reconstruction RC of the scout image in the 0 degree direction is performed.
In Step H4, reprojection processing is performed on the tomographic images continuous in the Z direction in the direction of the view angle of 90 degrees in the x-axis direction, and image reconstruction RC of the scout image in the direction of 90 degrees is performed.

  The tomographic images continuous in the Z direction are a tomographic image with an X-ray tube voltage of 80 kV, a tomographic image with an X-ray tube voltage of 140 kV, a tomographic image of a calcium-weighted image, and a tomographic image of a contrast-agent-weighted image. A scout image in the 0-degree direction or 90-degree direction is created by the following method.

For example, in the case of a scout image with an X-ray tube voltage of 140 kV, the following (Equation 15) and (Equation 16) are obtained. Note that a tomographic image with an X-ray tube voltage of 80 kV continuous in the Z direction is G80 (x, y, z), and a tomographic image with an X-ray tube voltage of 140 kV is G140 (x, y, z). The reprojection profile data in the 0 degree direction (Y direction) is Py (x, y), and the reprojection profile data in the 90 degree direction (X direction) is Px (y, z). Here, N is the pixel size N × N of the tomographic image.
. . . (Formula 15)
. . . (Formula 16)

Similarly, in the case of a scout image with an X-ray tube voltage of 80 kV, the following (Equation 17) and (Equation 18) are obtained.
. . . (Formula 17)
. . . (Formula 18)

In the case of a scout image of a contrast-enhanced image, the following (Formula 19) and (Formula 20) are obtained. In the case of a scout image of a calcium-weighted image, the following (Formula 21) and (Formula 22) are obtained. become. Note that the contrast-enhanced image is Gio (x, y, z), and the calcium-enhanced image is Gca (x, y, z).
. . . (Formula 19)
. . . (Formula 20)
. . . (Formula 21)
. . . (Formula 22)

  That is, the scout image SC can be created by continuously arranging the profiles Py (x, z) and Px (y, z) in the Z direction.

In step H5, a 0 degree direction scout image and a 90 degree direction scout image at one X-ray tube voltage are displayed as images. The operator can set shooting conditions by referring to various scout images. In this embodiment, the reprojection process is performed with the direction of the scout image being 0 degrees and 90 degrees, but may be 180 degrees, 270 degrees, or any angle.
According to the present embodiment, the X-ray control unit 22 of the X-ray CT apparatus 100 uses the X-ray projection data obtained by performing the helical scout scan while switching to a plurality of X-ray tube voltages, thereby correcting the X-ray tube voltage. It is possible to obtain position information of a substance contained in a necessary subject and to set appropriate imaging conditions at the time of the main scan. The image reconstruction unit 34 can obtain a tomographic image having a uniform X-ray tube voltage between the tomographic images by correcting the obtained tomographic image with the X-ray tube voltage.

(Embodiment 2)
In the present embodiment, an example of setting a noise index value, which is an image quality index value for each Z-direction coordinate range, in setting the shooting conditions will be described.

  In other words, the X-ray control unit 22 preferentially controls the X-ray tube current value, and changes the X-ray tube voltage value when the set range of the X-ray tube current value is exceeded. For example, when the X-ray controller 22 captures images from the neck to the liver as shown in FIG. 5B, the X-ray tube current value reaches the maximum X-ray tube current value mAMAX near the shoulder. In such a case, the X-ray tube voltage value is increased for imaging. In such a case, the shooting condition can be set with the noise index value.

FIG. 10 shows a flowchart for setting the photographing condition with the noise index value.
Steps D21 to 23 are substantially the same as those in the first embodiment, and thus description thereof is omitted.

  In step D24, shooting conditions for the main scan are set. The Z direction range 1 specifies the noise index value NI1, the Z direction range 2 specifies the noise index value NI2, and the Z direction range 3 specifies the noise index value NI3. Note that i = 1. The noise index value is obtained from the scout image of the X-ray transmission profile data of the subject from the 0 degree direction and the 90 degree direction at the imaging position in the Z direction.

  In Step D25, the X-ray tube current value mA in the Z direction range i is obtained from the geometric feature amount of the scout image. The X-ray control unit 22 determines the X-ray tube current value mA so that the noise of the final tomographic image can be closer to the noise index value based on the noise index value designated for each Z-direction coordinate position.

In step D26, it is determined whether there is a place where the maximum X-ray tube current mAMAX is reached. If YES, the process proceeds to step D27, and if NO, the process proceeds to step D32.
In step D27, the imaging of the main scan is performed under the imaging conditions of the set X-ray tube voltage kV and X-ray tube current mA.
In Step D28, it is determined whether or not the X-ray tube voltage is constant. If YES, the process goes to Step D29, and if NO, the process goes to Step D33.

In step D29, the image reconstruction unit 34 performs image reconstruction.
In step D30, the tomographic image is displayed on the monitor 6.
In step D31, it is determined whether i = 3. If YES, the process ends. If NO, the process goes to step D34. In this flowchart, noise index values are set for three Z-direction ranges.

In step D32, the X-ray tube voltage value is changed. Thereafter, the process returns to Step D25.
In step D33, the X-ray tube voltage is corrected. Thereafter, the process returns to Step D29.
In step D34, i = i + 1. Thereafter, the process returns to Step D25.

  The X-ray control unit 22 changes the imaging condition with priority on the X-ray tube current, but may change the imaging condition with priority on the X-ray tube voltage. In steps D28 and D33, the X-ray tube voltage is corrected for a portion where the X-ray tube voltage is not constant.

(Embodiment 3)
In the present embodiment, instead of obtaining the dual energy ratio tomographic image in order to acquire the position information of each substance included in the subject in step D5 described in the first embodiment, It is an example which reconstructs a dual energy image and acquires position information of each substance contained in a subject.
FIG. 11 is a conceptual diagram for reconstructing an equivalent image (dual energy image) of an arbitrary substance M by weighted addition processing of tomographic images.

In the dual energy image reconstruction method, for example, when a tomographic image with an X-ray tube voltage of 80 kV is G80 (x, y) and a tomographic image with an X-ray tube voltage of 140 kV is G140 (x, y), FIG. As shown, the weighted addition coefficient w1 and the weighted addition coefficient w2 are used to perform the weighted addition processing as shown in the following (Equation 23). In the equivalent image GM (x, y) of the arbitrary substance M, the weighted addition coefficient w1 and the weighted addition coefficient w2 are adjusted so that the pixel value of the area of the arbitrary substance M becomes “0”. C is a constant for adjusting the bias value.
. . . (Formula 23)
However, when w1 = −w2, the following (Formula 24) is assumed.
. . . (Formula 24)
Also, since w2 is usually a negative number, it can be written as in (Equation 25) below.
. . . (Formula 25)
However, when k = 1, the equation (24) is obtained.

  That is, the equivalent image GM (x, y) of the arbitrary substance M is weighted and added to the tomographic image G80 (x, y) with the X-ray tube voltage 80 kV and the tomographic image G140 (x, y) with the X-ray tube voltage 140 kV. Can be created.

  Similarly, the equivalent image GM (x, y) of the arbitrary substance M can be created by weighted addition processing of X-ray projection data with an X-ray tube voltage of 80 kV and X-ray projection data with an X-ray tube voltage of 140 kV. And the positional information of each substance contained in the subject can be acquired by associating the substance image emphasized in each equivalent image with the pixel.

(Embodiment 4)
In the present embodiment, an example of using the dual energy ratio tomogram instead of the scout image as the method of obtaining the X-ray transmission profile data obtained in step D25 described in the third embodiment will be described. . That is, more accurate water-equivalent X-ray profile data at the desired Z-direction coordinate position is obtained by converting the X-ray profile of each dual energy ratio range into a water-equivalent X-ray profile, and in all dual energy ratio ranges. It can be obtained by cumulative addition.

FIG. 12 shows a flowchart for obtaining a water-equivalent X-ray profile from the dual energy ratio tomogram at the Z-direction coordinate position.
In step D251, a continuous dual energy ratio tomogram in the Z direction is input.
In step D252, the dual energy ratio der = 0.

In step D253, an X-ray transmission profile of pixels in the range [der, der + 0.1] is obtained.
In step D254, the X-ray transmission profile of the pixels in the range [der, der + 0.1] is converted into a water-equivalent X-ray profile.
In step D255, a water-equivalent X-ray profile in the range [der, der + 0.1] is stored.

  In step D256, it is determined whether or not der> 3.0. If YES, the process proceeds to step D257, and if NO, the process proceeds to step D258.

In step D257, the water equivalent X-ray profile data obtained in steps D253 to D255 are all added in the range of der [0.0, 3.0], and the water equivalent X-ray profile at the Z-direction coordinate position is added. It is said.
In step D258, der = der + 0.1 is set. Thereafter, the process returns to Step D253.

(Embodiment 5)
FIG. 13 is a diagram for explaining the operation of the X-ray CT apparatus according to the present embodiment. Note that the X-ray CT apparatus used in the present embodiment is the same as that in the first embodiment, and therefore the description of the entire structure is omitted.

In step D41, the subject is placed on the cradle 12 of the imaging table 10, and alignment is performed.
In step D42, X-ray data collection is performed by performing a scout scan with an X-ray tube voltage of 120 kV. A general scout imaging in which the gantry rotating unit 15 is not rotated may be performed, and imaging may be performed from one direction with a single tube voltage.
In step D43, a scout image is displayed on the monitor 6. A scout image in the 0 degree direction or the 90 degree direction shown in FIG. 14A is displayed by the scout scan.

  In step D44, the X-ray controller 22 obtains the X-ray tube voltage for dual energy imaging of the main scan for each view angle at each coordinate position in the Z direction based on the profile area of the scout image. As shown in FIG. 14B, an X-ray tube voltage of 80 kV and an X-ray tube voltage of 120 kV is set at the Z-direction coordinate position with a small profile area, and the Z-direction coordinate position with a large profile area is set. The X-ray tube voltage is set to an X-ray tube voltage of 140 kV and an X-ray tube voltage of 120 kV, thereby reducing the exposure of the subject. That is, in the range of [z1, z2] shown in FIG. 13A, the X-ray tube voltage is set to the X-ray tube voltage 80 kV and the X-ray tube voltage 140 kV.

In step D45, X-ray data collection is performed while switching between a low X-ray tube voltage and a high X-ray tube voltage for each view or for each of a plurality of views.
In step D46, a view of X-ray projection data having a high X-ray tube voltage is extracted, and a view of X-ray projection data having a low X-ray tube voltage is extracted. The X-ray projection data extraction method uses the same X-ray projection data extraction method as that described with reference to FIG.

  In step D47, the X-ray projection data of the substance A and the substance B is obtained by performing a weighted addition process on the X-ray projection data having a high X-ray tube voltage and the X-ray projection data having a low X-ray tube voltage. For example, the substance A and the substance B may be a combination of contrast medium Io and water, contrast medium Io and calcium Ca, and calcium Ca and water. As the weighted addition process, a first-order weighted addition process, a weighted addition process of a plurality of orders, and a non-linear correction process may be performed simultaneously.

  In Step D48, a tomographic image of the substance A and the substance B is reconstructed. The image reconstruction unit 34 reconstructs the density tomographic images of the substances A and B using the z filter convolution process, the reconstruction function convolution process, the three-dimensional backprojection process, and the post-process.

  In step D49, X-ray absorption of two substances is performed for a certain tube voltage, for example, a tomographic image corresponding to an X-ray tube voltage of 80 kV, a tomographic image corresponding to an X-ray tube voltage of 140 kV, a tomographic image corresponding to an X-ray tube voltage of 120 kV. It can be obtained from the coefficient and the density tomogram of the two substances.

  In step D50, the above-described tomographic image is displayed. The image reconstruction unit 34 includes a plurality of tomographic images of the substance A and the substance B, such as a tomographic image corresponding to an X-ray tube voltage of 80 kV, a tomographic image corresponding to an X-ray tube voltage of 140 kV, and a tomographic image corresponding to an X-ray tube voltage of 120 kV. The tomographic images can be reconstructed so that a uniform X-ray tube voltage tomographic image can be obtained between the tomographic images. The image reconstruction unit 34 reconstructs a dual energy ratio tomographic image from a tomographic image with an X-ray tube voltage of 80 kV, a tomographic image with an X-ray tube voltage of 120 kV, and an X-ray tube voltage of 140 kV, and a calcium-weighted tomographic image. Dual energy imaging tomograms such as contrast-enhanced tomograms can also be reconstructed.

The present invention is not limited to the above embodiment.
Although the above embodiment describes the case where the scanning gantry 20 is not tilted, the same effect can be obtained even in the case of so-called tilt scanning in which the scanning gantry 20 is tilted. Moreover, although this embodiment has described the case where X-ray data acquisition is not synchronized with a biological signal, the same effect can be obtained even when synchronized with a biological signal, particularly a heartbeat signal.
In the above embodiment, an X-ray CT apparatus having a two-dimensional X-ray area detector having a matrix structure represented by a multi-row X-ray detector or a flat panel X-ray detector is described. The same effect can be obtained in the X-ray CT apparatus of the row X-ray detector.

In the above embodiment, helical scanning or the like is realized by moving the cradle 12 of the imaging table 10 in the Z direction. However, relatively similar effects can also be obtained by moving the scanning gantry 20 or the rotating portion 15 in the scanning gantry 20 with respect to the cradle 12 of the imaging table 10.
In the above embodiment, the medical X-ray CT apparatus is described based on the original, but also in an industrial X-ray CT apparatus or an X-ray CT-PET apparatus, an X-ray CT-SPECT apparatus combined with other apparatuses, etc. Available.

1 is a configuration block diagram of an X-ray CT apparatus 100 according to an embodiment of the present invention. It is a flowchart which shows the outline | summary of operation | movement about the X-ray CT apparatus 100 which concerns on embodiment of this invention. (A) is a figure which shows the method of performing a helical scan, switching an X-ray tube voltage for every view. (B) is a figure which shows the method of performing a helical scan, switching an X-ray tube voltage for every several view. It is the graph which showed the dual energy ratio by a different X-ray tube voltage. (A) is a figure which shows the imaging | photography range of the scout image of a 90 degree | times direction. (B) is a figure which shows the change of the X-ray tube voltage and X-ray tube current for every imaging | photography range. It is a figure which shows the view angle range of the shoulder vicinity. (A) is a figure which shows the selection screen of fixed tube voltage mode FixkV and view tube voltage mode ViewkV. (B) is a figure which shows the selection screen of fixed tube current mode FixmA and view tube current mode ViewmA. It is a figure which shows the extraction method of X-ray projection data, and the interpolation method. It is a flowchart of the image reconstruction in a helical scout scan. It is a flowchart which sets an imaging condition with a noise index value. It is a conceptual diagram which reconstructs the equivalent image of the arbitrary substance M by the weighted addition process of a tomogram. It is a flowchart which calculates | requires a water equivalent X-ray profile from a dual energy ratio tomogram. It is a flowchart which shows the outline | summary of operation | movement about the X-ray CT apparatus 100 which concerns on embodiment of this invention. (A) is a figure which shows the imaging | photography range of the scout image of a 90 degree | times direction. (B) is a figure which shows the change of the different X-ray tube voltage and X-ray tube current for every imaging | photography range.

Explanation of symbols

1 ... Operation console 1
2 ... Input device 3 ... Central processing unit 5 ... Data collection unit 6 ... Monitor 7 ... Storage device 10 ... Imaging table 12 ... Cradle 15 ... Rotating unit 20 ... Scanning gantry 21 ... X-ray tube 22 ... X-ray control unit 23 ... Collimator 24 ... Multi-row X-ray detector 25 ... Data acquisition device (DAS)
26 ... Rotation control unit 28 ... Beam forming X-ray filter 29 ... Gantry control unit 31, 31 '... X-ray tube voltage detector 33 ... Pre-processing unit 34 ... Image reconstruction unit 100 ... X-ray CT apparatus Ca ... Calcium Io ... Contrast medium HB ... Subject SC ... Scout image

Claims (6)

An X-ray CT apparatus having an X-ray tube,
When the subject is scanned at a predetermined tube voltage based on the scout data obtained by the dual energy imaging of the helical scout scan, the noise index of the tomographic image at each position in the body axis direction of the subject A calculation unit that calculates a tube current at each position and each view angle in the body axis direction when performing the scan so that the value approaches a designated index value;
Of each position and each view angle in the body axis direction, the range where the calculated tube current is less than or equal to the allowable maximum current is scanned with the predetermined tube voltage and the calculated tube current, For the range in which the calculated tube current exceeds the allowable maximum current, the X-ray tube is controlled so that scanning is performed at a tube voltage higher than the predetermined tube voltage and a tube current below the maximum allowable current. A line control unit;
Projection data acquired at a tube voltage higher than the predetermined tube voltage is corrected to projection data obtained at the predetermined tube voltage based on the scout data, and at least one of the positions is corrected. for One position, reconstructing an image by using the said corrected projection data acquired by the projection data and the predetermined tube voltage was equivalent to the tomographic image at the position of the subject by the predetermined tube voltage An X-ray CT apparatus.   The X-ray control unit determines the tube current of the X-ray tube while fixing the tube voltage of the X-ray tube to the predetermined tube voltage for a range where the calculated tube current is equal to or less than the allowable maximum current. The range of the calculated tube current exceeding the allowable maximum current is adjusted, and the tube voltage of the X-ray tube is adjusted while fixing the tube current of the X-ray tube to the allowable maximum current. X-ray CT apparatus described in 1. The image reconstruction unit is irradiated with information on a substance in a subject portion irradiated with X-rays with a tube voltage higher than the predetermined tube voltage obtained by the scout data, and with X-rays with the predetermined tube voltage. A tube higher than the predetermined tube voltage based on the X-ray absorption coefficient of the material at the time of irradiation and the X-ray absorption coefficient of the material when irradiated with X-rays with a tube voltage higher than the predetermined tube voltage. The X-ray CT apparatus according to claim 1, wherein the X-ray CT apparatus corrects projection data acquired with a voltage. The image reconstruction unit converts projection data at a tube voltage higher than the predetermined tube voltage into data corresponding to the projection data at the predetermined tube voltage by a conversion coefficient using an X-ray absorption coefficient of the substance. The X-ray CT apparatus according to claim 3, wherein conversion is performed .   The X-ray CT apparatus according to any one of claims 1 to 4, wherein the allowable maximum current is an upper limit value of a settable range.   The X-ray CT apparatus according to claim 1, wherein the predetermined tube voltage is substantially 120 kV.