JP3673041B2 - CT imaging method and X-ray CT apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a CT (Computed Tomography) imaging method and an X-ray CT apparatus, and more particularly, a CT imaging method that can reduce calculation time and does not require a user to perform a pre-scan using a reference phantom (fantom). And to an X-ray CT apparatus.
[0002]
[Prior art]
In a conventional CT imaging method, pre-scanning is performed using a reference phantom in which water is put in a cylindrical container, and reference projection data is obtained. Then, the reference projection data is subtracted from the projection data acquired by scanning the subject, and a CT image is created by performing filtering processing and back projection processing on the subtracted data.
[0003]
Next, a conventional CT imaging method will be described in detail with reference to FIGS.
FIG. 9 is a flowchart showing an example of the pre-scan procedure.
In step C1, the reference phantom is scanned to collect raw data and subjected to sensitivity correction and logarithmic calculation to obtain reference projection data Ki (i: detector channel number). FIG. 10 schematically shows the reference projection data Ki.
[0004]
FIG. 11 is a flowchart showing an example of the procedure of the main scan.
In step D1, an X-ray tube (or X-ray tube and detector) is rotated around the subject to collect raw data at different view angles.
In step D2, sensitivity correction and logarithmic calculation are performed on the raw data to obtain projection data Proj _ij (i: detector channel number, j: view angle number). FIG. 12 schematically shows the projection data Proj _ij .
In step D3, the reference projection data Ki is subtracted from the projection data Proj _ij to calculate differential projection data P′roj _ij . FIG. 13 schematically shows the differential projection data P′roj _i1 of the first view.
[0005]
In step D4, the differential projection data P′roj _ij is subjected to FFT (Fast Fourier Transform) and converted to frequency domain data. Here, the number of FFT points is set to a power number N of 2 which is sufficiently larger than the number of data (number of detector channels). For example, when the number of data is 1500, the FFT point number N is 4096.
In step D5, the frequency domain data is multiplied by a reconstruction function.
In step D6, IFFT (Inverse Fast Fourier Transform) is performed on the frequency domain data multiplied by the reconstruction function to return to projection data. Here, the IFFT score is the same as the FFT score N.
The steps D4 to D6 are filtering processes.
[0006]
In step D7, a back projection operation is performed on the projection data to create a CT image.
In step D8, a CT image is displayed.
[0007]
[Problems to be solved by the invention]
In the above-described conventional CT imaging method, if the number of FFT points N in the filtering process is reduced, cupping is performed on the image (in spite of the fact that the CT value should be a uniform CT value, the CT value is converted to the CT value at the periphery and the center). In the case of a circular region, a difference occurs, resulting in an artifact that looks as if a cup has been placed), so it was necessary to increase the number of FFT points N sufficiently. However, if the number of FFT points N is sufficiently large, there is a problem that it takes a long calculation time. There is also a problem that pre-scanning using a reference phantom must be performed.
SUMMARY OF THE INVENTION An object of the present invention is to provide a CT imaging method and an X-ray CT apparatus that can shorten the calculation time and do not require pre-scanning using a reference phantom.
[0008]
[Means for Solving the Problems]
In the first aspect, the present invention holds ideal data of a plurality of ideal phantoms having different sizes, scans the subject to acquire data, and obtains the ideal of the ideal phantom having a size closest to the size of the subject. Provided is a CT imaging method characterized in that data is taken out and subtracted from the data, and a CT image is created by performing filtering processing and back projection processing on the data after the subtraction.
The inventors of the present invention diligently studied that the cause of cupping on the image when the number of FFT points N in the filtering process is reduced in the conventional CT imaging method is the difference projection data P′roj _i1 (FIG. 13). This is because the influence of the low frequency component remaining on the surface is so large that it cannot be ignored. I found out.
Therefore, in the CT imaging method of the first aspect, ideal data that would be obtained by scanning a plurality of ideal phantoms having different sizes is calculated and stored by simulation, and the size closest to the size of the subject is measured. The ideal data of the ideal phantom was selected and subtracted from the data obtained by scanning the subject. In this way, if ideal data of an ideal phantom with a small difference in size from the subject is used, low frequency components having an influence that cannot be ignored are not left in the difference data, so the number of FFT points in the filtering process is reduced. However, no cupping occurs on the CT image. Therefore, the calculation time can be shortened. Further, since it is sufficient to calculate ideal data by simulation, pre-scanning using a reference phantom becomes unnecessary.
An ideal phantom of a substance other than water may be assumed. In this case, in order to match the CT value, a constant value may be added to the result of the back projection process (the CT value is a value based on water).
[0009]
In a second aspect, the present invention provides a CT imaging method characterized in that, in the CT imaging method configured as described above, ideal data is calculated by simulation assuming an ideal phantom having an elliptical cross section.
When viewed from the X-ray CT apparatus, the cross-sectional shape of the subject (human body) is closer to an ellipse than a circle. Therefore, in the CT imaging method of the second aspect, an ideal phantom having an elliptical cross section is assumed. If the elliptical cross section is used, the difference in size from the human body is smaller than that of the circular cross section, and low frequency components having an influence that cannot be ignored are not left in the differential projection data. Therefore, the calculation time can be shortened as described in the first aspect.
If a circular cross section is used, a low frequency component remains in the differential projection data and the image quality deteriorates. However, since it is not necessary to consider the view angle, the amount of ideal data can be reduced. Therefore, an ideal phantom having a circular cross section may be assumed if a reduction in image quality can be tolerated.
[0010]
In a third aspect, the present invention provides an ideal data holding unit that holds ideal data obtained by scanning a plurality of ideal phantoms having different sizes, a scanning unit that scans a subject and collects data, and the holding Ideal data selection means for selecting ideal data of an ideal phantom having a size closest to the size of the subject from among the ideal data, subtraction means for subtracting the selected ideal data from the data, and data after the subtraction There is provided an X-ray CT apparatus comprising filtering processing means for performing filtering processing on the image data and back projection processing means for performing back projection processing on the data after the filtering processing.
According to the X-ray CT apparatus of the third aspect, the CT imaging method according to the first aspect can be suitably implemented.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to embodiments shown in the drawings. Note that the present invention is not limited thereby.
[0012]
FIG. 1 is a configuration diagram of an X-ray CT apparatus according to an embodiment of the present invention.
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 that receives an operator's instructions and information, a central processing unit 3 that executes a scan process, a filtering process, a back projection process, and the like. A control interface 4 that outputs data, a data collection buffer 5 that collects data acquired by the scanning gantry 20, a CRT 6 that displays images, and a storage device 7 that stores various data and programs such as ideal data. doing.
The imaging table 10 carries a subject and moves it in the body axis direction.
The scanning gantry 20 includes a fan beam type X-ray tube 21, a collimator 22 and a detector 23, a rotation controller 24 for rotating the X-ray tube 21 and the detector 23 around the body axis of the subject, and an X-ray. An X-ray controller 25 for adjusting the timing and intensity of irradiation and a data collection unit 26 are provided.
[0013]
FIG. 2 is a flowchart showing ideal data creation processing. This ideal data creation process may be performed by the manufacturer of the X-ray CT apparatus 100 or by the user. When the manufacturer does, the obtained ideal data is provided to the user. On the other hand, when the user performs, an ideal data creation processing program is provided from the manufacturer to the user.
In step A1, ideal projection data R _{ij, s} is calculated by simulation assuming cross-sectional elliptic phantoms PHs (s: size number) having different sizes. The substance of phantom PHs is water. FIG. 3 conceptually shows an X-ray tube 21 ′, a detector 23 ′, and a cross-sectional elliptic phantom PHs on simulation. FIG. 4 conceptually shows the ideal projection data R _{i1, s} of the first view. Lc is the width of the ideal projection data R _{i1, s} , and Lh is the height of the ideal projection data R _{i1, s} .
In step A2, the ideal projection data R _{ij, s} is stored in an ideal file in association with the width Lc, the height Lh, and the size number of the cross-sectional ellipse phantom PHs. FIG. 5 illustrates a correspondence table of the width Lc, the height Lh, and the size number of the ideal projection data R _{ij, s} .
[0014]
FIG. 6 is a flowchart showing an example of the procedure of the main scan.
In Step B1, the X-ray tube 21 and the detector 23 are rotated around the subject to collect raw data at different view angles.
In step B2, sensitivity correction and logarithmic calculation are performed on the raw data to obtain projection data Proj _ij . FIG. 7 schematically shows the projection data Proj _i1 of the first view. Lc is the width of the projection data Proj _i1 , and Lh is the height of the projection data Proj _i1 .
In step B3, the correspondence table (FIG. 5) is examined, and if there is data equal to the width Lc and height Lh of the projection data Proj _i1 of the first view, the ideal projection data R _{ij, S} corresponding to the size number is obtained. Extract from the ideal file. If there is no equivalent, the ideal projection data R _ij, S corresponding to the size number of the projection data Proj _i1 larger than the width Lc, height Lh and closest is extracted from the ideal file.
[0015]
In step B4, the extracted ideal projection data R _{ij, S} is subtracted from the projection data Proj _ij to calculate differential projection data P′roj _ij . FIG. 8 schematically shows the differential projection data P′roj _i1 of the first view. As can be seen from a comparison between FIG. 8 and FIG. 9, almost no low frequency component remains in the differential projection data P′roj _i1 of FIG.
[0016]
In step B5, the differential projection data P′roj _ij is subjected to FFT (Fast Fourier Transform) and converted to frequency domain data. Here, the number of FFT points is a power of 2 N / 2, which is larger than the number of data (number of detector channels). For example, when the number of data is 1500, the number of FFT points N is 2048 (half of the conventional value).
In step B6, the frequency domain data is multiplied by a reconstruction function.
In step B7, IFFT (Inverse Fast Fourier Transform) is performed on the frequency domain data multiplied by the reconstruction function to return to projection data. Here, the IFFT score is the same as the FFT score N / 2.
The steps B5 to B7 are filtering processes.
[0017]
In step B8, a back projection operation is performed on the projection data to create a CT image.
In step B9, a CT image is displayed.
[0018]
According to the X-ray CT apparatus 100 described above, ideal projection data R _{ij, s} that would be obtained by scanning a plurality of ideal phantoms having different sizes are calculated and stored by simulation, and the size of the subject is determined. The ideal projection data R _{ij, s} of the ideal phantom of the closest size is selected and subtracted from the projection data Proj _ij data obtained by scanning the subject. Data P'roji _j never remains. For this reason, even if the number of FFT points in the filtering process (steps B5 to B7) is reduced to half that of the prior art, cupping does not occur on the CT image. Therefore, calculation time can be shortened. In addition, since the ideal projection data R _{ij, s} is calculated by simulation, pre-scanning using the reference phantom is not necessary.
[0019]
In the above embodiment, when selecting the ideal projection data R _{ij, s} of the ideal phantom having the size closest to the size of the subject, the width Lc and the height Lh are obtained from the projection data Proj _i1 of the first view. The width Lc may be obtained from the scout image at the angle of the X-ray tube 21 of the view, and the height Lh may be obtained from the scout image at an angle different from that by 90 °.
[0020]
【The invention's effect】
According to the CT imaging method and the X-ray CT apparatus of the present invention, ideal data that would be obtained by scanning a plurality of ideal phantoms having different sizes is calculated and stored by simulation, and is most suitable for the size of the subject. Since the ideal data of the ideal phantom of a close size was selected and subtracted from the data acquired by scanning the subject, low frequency components that have a large influence that cannot be ignored will not remain in the difference data, Even if the number of FFT points in the filtering process is reduced, cupping does not occur on the CT image. Therefore, the calculation time can be shortened. Further, since it is sufficient to calculate ideal data by simulation, pre-scanning using a reference phantom becomes unnecessary.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing an X-ray CT apparatus according to an embodiment of the present invention.
FIG. 2 is a flowchart showing an ideal data creation process.
FIG. 3 is an explanatory diagram of a simulation for calculating ideal data.
FIG. 4 is an explanatory diagram of ideal projection data of a first view.
FIG. 5 is a diagram illustrating an example of a correspondence table of width, height, and size number of ideal projection data.
6 is a flowchart showing a main scan process by the X-ray CT apparatus of FIG. 1; FIG.
FIG. 7 is an explanatory diagram of projection data of a first view.
FIG. 8 is an explanatory diagram of differential projection data of a first view.
FIG. 9 is a flowchart of an example of a conventional prescan process.
FIG. 10 is an explanatory diagram of reference projection data.
FIG. 11 is a flowchart showing a main scan process performed by a conventional X-ray CT apparatus.
FIG. 12 is an explanatory diagram of projection data of a first view.
FIG. 13 is an explanatory diagram of differential projection data of a conventional first view.
[Explanation of symbols]
100 X-ray CT device 2 Input device 3 Central processing unit 7 Storage device 10 Imaging table 20 Scanning gantry 21 X-ray tube 23 Detector

Claims (3)

Holds ideal data of multiple ideal phantoms of different sizes, scans the subject to acquire data, takes the ideal data of the ideal phantom of the size closest to the size of the subject and subtracts it from the data, A CT imaging method, wherein a filtering process and a back projection process are performed on the data after the subtraction to create a CT image. 2. The CT imaging method according to claim 1, wherein ideal data is calculated by simulation assuming an ideal phantom having an elliptical cross section. Ideal data holding means for holding ideal data of a plurality of ideal phantoms of different sizes, scanning means for scanning the subject and collecting data, and the closest ideal data to the size of the subject among the held ideal data Ideal data selection means for selecting ideal data of an ideal phantom of a size, subtraction means for subtracting the selected ideal data from the data, filtering processing means for performing filtering processing on the data after the subtraction, and the filtering processing An X-ray CT apparatus comprising back projection processing means for performing back projection processing on subsequent data.

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