JP2006334319A - X-ray ct apparatus and method of preprocessing thereof, data preparation device and method thereof, and control program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus by which a precise projection image can be collected easily and inexpensively by eliminating the need for production and imaging of a water phantom. <P>SOLUTION: Projection image data of the water phantom is prepared by simulation, and water correction is carried out using this. Namely, water data for carrying out water correction is prepared by computer simulation by assuming the water phantom entirely filled with water virtually. The simulation is carried out considering affection of shading, beam hardening, scattered radiation, etc. due to irregularity of X-ray generation or the like at an X-ray source. The water data obtained like this serves as ideal water data from which noise is eliminated. Since water correction is carried out with the ideal data, a precise CT value is obtained. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention provides an X-ray CT apparatus and its preprocessing method used for medical use, a data creation apparatus and method for creating data used in the X-ray CT apparatus, and the preprocessing method. Related to the control program.

  An X-ray CT apparatus that transmits X-rays generated from an X-ray source to a patient as a subject and reconstructs the X-rays detected by the X-ray detector as a projection image has been generally known. It has been.

  In this type of X-ray CT apparatus, pre-processing (calibration) is performed using actual projection data in order to calibrate pixel values of a reconstructed image and to correct sensitivity between X-ray detectors. The projection data obtained by actually photographing the subject is corrected with the correction data obtained by this preprocessing.

  FIG. 3 is a flowchart showing a flow of main preprocessing in the conventional X-ray CT apparatus.

  First, in step S101, offset correction is performed. That is, as shown in Expression <1>, the offset data offset (data taken without generating X-rays) acquired in advance is subtracted from the projection data Data obtained by shooting the subject. An offset correction value Data ′ is obtained.

  In the next step S102, gain correction is performed. That is, as shown in the following equation <2>, the Data ′ value is divided by the offset-corrected white data white (data taken without placing a subject under the same photographing conditions as when photographing the subject) to obtain a value Data. Ask for ''. The white data white is obtained in advance.

  In subsequent step S103, log conversion is performed. That is, as shown in the formula <3>, the Data ″ value is log-transformed to obtain a negative value Data ′ ″.

  In step S104, water correction is performed. That is, as shown in Equation <4>, water correction is performed by subtracting water data water subjected to offset correction, gain correction, and log conversion from the Data ′ ″ value.

  Here, pixel values of an image reconstructed in the X-ray CT apparatus are created as values called CT values (or Hans field values). The CT value is a value defined such that the water X-ray absorption coefficient value is 0 and the air X-ray absorption coefficient value is −1000. In order to convert the CT value into a value called the CT value, the water correction is performed. Has been done.

  Conventionally, water data for performing this water correction is acquired by photographing a water phantom containing water of the same size as the photographing region.

Further, Patent Document 1 discloses a technique for preparing water data by photographing a plurality of water phantoms having different sizes in advance and selecting optimal water data depending on an imaging region.
Japanese Patent Laid-Open No. 9-24043

  However, in order to photograph the water phantom, it is necessary to accurately arrange the water phantom in the photographing area. Therefore, the installation of the water phantom required delicate adjustment and was not an easy task. In particular, when using a cone beam X-ray CT apparatus that irradiates the X-ray beam not only in the X-axis and Y-axis directions but also in the Z-axis direction (body axis direction), the water phantom is also applied in the body axis direction. It becomes long and very large, and cannot be easily placed and photographed.

  In addition, as in the technique of Patent Document 1, if the optimum water data is selected depending on the imaging region, the above-described arrangement and imaging must be repeated many times, and a plurality of large water phantoms are created. The cost is very high because it must be done.

  The amount of projection image data of the water phantom photographed at all angles of 360 degrees for each photographing condition becomes enormous, and in order to hold this enormous amount of data, a very large storage capacity is required. .

  In view of the above-described conventional problems, the present invention eliminates the need for producing and photographing a water phantom, and can easily collect a high-accuracy projection image at low cost, its preprocessing method, and data. It is an object of the present invention to provide a creation device, its method, and a control program.

  In order to achieve the above object, the present invention includes an X-ray generation unit that generates X-rays and an X-ray detection unit that detects the X-rays that have passed through a subject, and each X-ray detection unit detects the X-rays. In the X-ray CT apparatus that acquires X-rays as a projection image, water correction for correcting the pixel value of the projection image to be a value based on the water X-ray absorption coefficient is performed based on the projection image data of the water phantom. The water correction unit is configured to generate projection image data of the water phantom by simulation and perform the water correction based on the simulation data.

  The present invention further includes an X-ray generation unit that generates X-rays and an X-ray detection unit that detects the X-rays transmitted through the subject, and projects the X-rays detected by the X-ray detection units. In a preprocessing method of an X-ray CT apparatus acquired as an image, a water phantom projection created by simulation is used to perform water correction for correcting the pixel value of the projection image to a value based on the water X-ray absorption coefficient. It has the water correction process performed using image data, It is characterized by the above-mentioned.

  The present invention further includes an X-ray generation unit that generates X-rays and an X-ray detection unit that detects the X-rays transmitted through the subject, and projects the X-rays detected by the X-ray detection units. A computer-readable control program for executing a pre-processing method of an X-ray CT apparatus acquired as an image so that a pixel value of the projection image becomes a value based on an X-ray absorption coefficient of water It is characterized by having a water correction step for performing the water correction to be corrected using the projection image data of the water phantom created by the simulation.

  Furthermore, the present invention is a data creation device for creating water correction data in an X-ray CT apparatus having an X-ray detection means for detecting X-rays generated by the X-ray generation means, A first input means for inputting respective coordinate data of the X-ray detection means; a second input means for inputting coordinate data of a water phantom for creating the water correction data; A storage means for storing the respective coordinate data input by the first input means and the second input means, and an operation for calculating the water correction data based on the respective coordinate data stored in the storage means And storage means for storing the data calculated by the calculation means.

  The present invention also provides a data creation method for creating water correction data in an X-ray CT apparatus having an X-ray detection means for detecting X-rays generated by the X-ray generation means, A first input step for inputting coordinate data of each of the X-ray detection means, a second input step for inputting coordinate data of a water phantom for creating the water correction data, and A storage step of storing the respective coordinate data input by the first input unit and the second input unit in the storage unit, and the water correction data based on the respective coordinate data stored in the storage unit It has a calculation step for calculating, and storage means for storing the data calculated in the calculation step.

  According to the present invention, it becomes unnecessary to produce and shoot a water phantom, and it is possible to easily collect highly accurate projection images.

  Embodiments of an X-ray CT apparatus and its preprocessing method, a data creation apparatus and method, and a control program according to the present invention will be described with reference to the drawings.

<Configuration of X-ray CT apparatus>
FIG. 1 is a block diagram showing an outline of the overall configuration of an X-ray CT apparatus according to an embodiment of the present invention.

  The X-ray CT apparatus includes an X-ray detector 1, an X-ray source 3, an X-ray generator control unit 4, and an image input unit 5, and the X-ray source 3 controlled by the X-ray generator control unit 4. The generated X-ray passes through the patient 2 as the subject and is detected by the X-ray detector 1. The detected X-ray is input to the image input unit 5 as a projection image.

  The X-ray source 3 and the X-ray detector 1 collect projection images for each predetermined rotation angle while rotating around the patient 2 as a rotation center. Alternatively, the patient 2 as the subject fixed to the rotary table may be rotated while maintaining the positional relationship between the X-ray source 3 and the X-ray detector 1.

  Further, the X-ray CT apparatus includes an image processing unit 7, an image storage unit 8, a diagnostic monitor 9, and an operation unit 10, and projection images at respective rotation angles input to the image input unit 5 are image processing units 7. Thus, image processing such as correction of the X-ray detector 1, preprocessing including log conversion, reconstruction processing, and the like is performed, and a tomographic image group is created. The created tomographic image group is displayed on the diagnostic monitor 9, stored in the image storage unit 8, or output to the printer 12, the diagnostic workstation 13, or the image database 14 via the network 11.

  Various operations such as a display window operation, a tomographic image switching display operation in the body axis direction, a cross-section conversion operation, and a three-dimensional surface display operation are performed by the operation unit 10. Then, the system control unit 6 controls these entire operations.

<Water data creation process according to the present embodiment>
In the X-ray CT apparatus configured as described above, the preprocessing of the present embodiment is executed by the image processing unit 7 in accordance with the flow of FIG. 3 described above, but for execution, offset data, white data, water Prepare data in advance. In this case, the feature of the present embodiment is that the water data is created by computer simulation using a computer assuming a water phantom that is virtually filled with water.

  FIG. 4 is a configuration diagram of a computer for obtaining water data according to the present embodiment by computer simulation.

  In FIG. 4, reference numeral 101 denotes a CPU, which controls the entire apparatus by executing a program stored in the ROM 103. The CPU 101 and each unit are connected by a system bus 107. Reference numeral 102 denotes a RAM which is used as a work area when the CPU 101 executes various data and programs. Reference numeral 103 denotes a ROM which stores a program for realizing the present embodiment, an initial setting value of the apparatus, and the like. A display unit 104 displays various information under the control of the CPU 101. An input unit 105 includes a keyboard, a mouse, and the like, and is operated by the user when performing various setting values for performing simulations, giving instructions to the apparatus, and the like. Reference numeral 106 denotes an external storage device that stores a value obtained as a result of the simulation in a storage medium. The value stored in the external storage device is set in the aforementioned X-ray CT apparatus. Note that a network interface may be provided, and the setting value may be transmitted to the X-ray CT apparatus via the network.

  In the present embodiment, it is assumed that the water phantom has a cylindrical shape corresponding to the imaging region, and this geometrical relationship is shown in FIG.

  FIG. 2 is an explanatory diagram for explaining the geometric relationship of the water phantom projection.

  First, the user operates the input unit 105 to set the coordinate value of the X-ray detector, the coordinate value of the X-ray source, and the cylindrical coordinate value that is a water phantom. This value is set based on the physical arrangement of the X-ray CT apparatus shown in FIG. In FIG. 2, the center of the cylindrical shape is the center of the coordinate system.

  Each coordinate value input by the input unit 105 is stored in the RAM 102, and the virtual space shown in FIG.

  Next, the CPU 101 performs the following calculation.

  That is, in the path from the X-ray source 3 to the point of the coordinates P (Py, Pz) of the X-ray detector 1, the length s2 passing through the cylindrical water phantom and the other air are passed. The length (s1 + s3) is obtained.

  The procedure for obtaining the length s2 is as follows: (1) First, an equation of the X-ray path (straight line SP) is obtained; (2) next, two intersection points of the straight line SP and the cylinder are obtained; (3) The procedure is to find the distance between the intersections. Hereinafter, the procedures (1), (2), and (3) will be described in order.

  (1) X-ray path (straight line SP) equation

Therefore, the equation of the X-ray path (straight line SP) is

It becomes.

(2) Calculation of two intersection points of straight line SP and cylinder The equation of the cylinder is

Therefore, the equation (a)

given that,

Because

  Therefore

  From this, the two intersections are

It becomes. Note that t1 is + in the formula (c) and t2 is-in the formula (c).
(3) Distance between two intersections The distance between the two intersections is

Can be obtained.

  As described above, the length s2 through which the X-ray projected on the point P (Py, Pz) passes through the water phantom is obtained. Here, if the total length of the X-ray path projected onto the point P is L, L is

And the length (s1 + s3) passing through the air is

If the X-ray intensity when there is no attenuation is I0, and the X-ray intensity when there is water and air is IW,

  Here, μA and μW are air and water X-ray absorption coefficients, respectively.

  Also, if the X-ray intensity when taking a picture without a water phantom (white image) is IA,

It is. Therefore,

It becomes.

  The projection image data of the water phantom by simulation is created using equations (d) and (e). For this purpose, first, the passage length s2 in the water phantom is obtained by substituting the pixel position coordinates P (Py, Pz) into the equation (d) for each pixel of the projection image.

  Then, this s2 is substituted into the equation (e). Where in equation (e)

The average pixel value of a white image taken without actually placing a subject may be used.

<Water correction according to this embodiment>
As described above, ideal water phantom data can be created. For water correction, the output level of each X-ray detector 1 is equivalent to the output level at the time of actual imaging. There is also an object of correcting the variation.

  Therefore, for example, by imaging a flat acrylic plate with an X-ray CT apparatus, it is possible to obtain variation data between pixels close to the output level at the time of actual imaging at the time of output of each X-ray detector 1. If this data is reflected in the projection image data of the water phantom by the above-mentioned simulation, more accurate simulation data can be created. The acrylic plate need not always be photographed at the time of calibration, and may be created using the correlation with the white image using data photographed at the time of installation of the apparatus.

  There is also a method of simulating shading of a white image using the square law of distance in order to create more precise simulation data from the projection image data of the water phantom by the simulation. For this shading, a value obtained by fitting the actual data of the white image with a low-dimensional function may be used.

  In this way, the projection image data of the water phantom by the simulation including the influence of shading under various photographing conditions can be created. However, in order to perform a more precise simulation, beam hardening (beam) The simulation may be performed in consideration of the effects of (hardening) and scattered radiation.

  The beam hardening simulation may be performed by obtaining a function of the output value of each X-ray detector 1 according to the thickness of water by experiment. In the simulation of scattering, scattering at the time of passing through the inside of water is simulated by a Monte Carlo method or the like. Alternatively, the scattering at the portion where the air hits the water with higher physical density from the air with lower physical density is also simulated by the Monte Carlo method or the like. Further, simulation using experimental data may be performed without using the Monte Carlo method.

  In addition, if simulation is performed using mathematical formulas and experimental data derived from various physical phenomena from the theory, it is possible to create projection image data that is inferior to the case of actual shooting.

  In this way, the projection image data (water data) of the water phantom is created by performing simulation using actually captured data (such as shading) and experimentally obtained data (such as scattering and beam hardening). By performing water correction using the water data, a reconstructed image with less artifacts can be obtained.

<Advantages of this embodiment>
In the present embodiment, projection image data of a water phantom is created by simulation, and water correction is performed using this. That is, the water data for performing water correction is created by computer simulation assuming a water phantom that is virtually filled with water. The simulation was performed in consideration of the effects of shading, beam hardening, scattered radiation, etc. due to X-ray generation unevenness of the X-ray source.

  The water data obtained in this way is ideal water data from which noise has been eliminated. Since water correction is performed using this ideal data, an accurate CT value can be obtained, artifacts can be suppressed, and highly accurate diagnosis can be performed. In addition, since there is no need to make and shoot a water phantom, it is possible not only to reduce costs but also to save unnecessary shooting. Furthermore, since the data is the same for all angles, only one angle of data need be created, and the storage capacity can be reduced, further reducing the cost.

  Note that an object of the present invention is to supply a storage medium storing software program codes for realizing the functions of the embodiments to a system or apparatus, and a computer (or CPU, MPU, or the like) of the system or apparatus as a storage medium. This can also be achieved by reading and executing the stored program code.

  In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the program code and the storage medium storing the program code constitute the present invention.

  Examples of the storage medium for supplying the program code include a floppy (registered trademark) disk, a hard disk, a magneto-optical disk, a CD-ROM, a CD-R, a CD-RW, a DVD-ROM, a DVD-RAM, and a DVD. -RW, DVD + RW, magnetic tape, nonvolatile memory card, ROM, etc. can be used. Alternatively, the program code may be downloaded via a network.

  Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an OS (operating system) running on the computer based on the instruction of the program code. A case where part or all of the actual processing is performed and the functions of the above-described embodiments are realized by the processing is also included.

  Further, after the program code read from the storage medium is written in a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. This includes a case where the CPU or the like provided in the board or the function expansion unit performs part or all of the actual processing, and the functions of the above-described embodiments are realized by the processing.

  In this case, the program is supplied by downloading directly from a storage medium storing the program or from another computer or database (not shown) connected to the Internet, a commercial network, a local area network, or the like.

It is a block diagram which shows the outline of the whole structure of the X-ray CT apparatus which concerns on embodiment. It is explanatory drawing for demonstrating the geometric relationship of water phantom projection. It is a flowchart which shows the flow of the main preprocessing in the conventional X-ray CT apparatus. It is a block diagram of the computer for calculating | requiring creation of the water data which concerns on embodiment by computer simulation.

Explanation of symbols

1 X-ray detector 3 X-ray source 4 X-ray generator control unit 6 System control unit 7 Image processing unit 8 Image storage unit 9 Diagnostic monitor

Claims (11)

X-rays having X-ray generation means for generating X-rays and X-ray detection means for detecting the X-rays that have passed through the subject, and acquiring the X-rays detected by the X-ray detection means as projection images In CT equipment,
Water correction means for performing water correction for correcting the pixel value of the projection image to be a value based on the water X-ray absorption coefficient based on the projection image data of the water phantom,
The water correction means is
An X-ray CT apparatus characterized in that projection image data of the water phantom is created by simulation and the water correction is performed based on the simulation data.   2. The simulation includes a simulation using variation data between pixels at the time of output of each X-ray detection unit that outputs a detection signal at a level approximate to an output level at the time of imaging. The X-ray CT apparatus described.   The X-ray CT apparatus according to claim 1, wherein the simulation includes a shading simulation.   The X-ray CT apparatus according to claim 1, wherein the simulation includes a beam hardening simulation.   The X-ray CT apparatus according to claim 1, wherein the simulation includes a simulation of scattered radiation.   The X-ray CT apparatus according to claim 1, wherein the simulation uses experimental data. X-rays having X-ray generation means for generating X-rays and X-ray detection means for detecting the X-rays that have passed through the subject, and acquiring the X-rays detected by the X-ray detection means as projection images In the pre-processing method of the CT apparatus,
A water correction step of performing water correction for correcting the pixel value of the projection image so as to be a value based on an X-ray absorption coefficient of water by using projection image data of a water phantom created by simulation. A preprocessing method for an X-ray CT apparatus. X-rays having X-ray generation means for generating X-rays and X-ray detection means for detecting the X-rays transmitted through the subject, and acquiring the X-rays detected by the X-ray detection means as projection images A computer-readable control program for executing a pre-processing method of a CT apparatus,
A water correction step for performing water correction for correcting the pixel value of the projection image so as to be a value based on an X-ray absorption coefficient of water using the projection image data of the water phantom created by the simulation is provided. Control program. A data creation device for creating water correction data in an X-ray CT apparatus having an X-ray detection means for detecting X-rays generated by an X-ray generation means,
First input means for inputting respective coordinate data of the X-ray generator and the X-ray detection means;
A second input means for inputting the coordinate data of the water phantom for creating the water correction data;
Storage means for storing respective coordinate data input by the first input means and the second input means;
Based on the respective coordinate data stored in the storage means, calculating means for calculating the water correction data;
And a storage means for storing the data calculated by the calculation means. The coordinate data input by the second input means is data indicating a cylindrical shape,
The data creation apparatus according to claim 9, wherein the calculation unit calculates the data based on a ratio of passing through the cylinder. A data creation method for creating water correction data in an X-ray CT apparatus having an X-ray detection means for detecting X-rays generated by an X-ray generation means,
A first input step for inputting respective coordinate data of the X-ray generator and the X-ray detector;
A second input step for inputting coordinate data of a water phantom for creating the water correction data;
A storage step of storing each coordinate data input by the first input means and the second input means in a storage means;
A calculation step for calculating the water correction data based on the respective coordinate data stored in the storage means;
And a storage means for storing the data calculated by the calculation step.

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