JP2008043820A - X-ray ct system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To determine a more accurate beam hardening (BH) correction coefficient. <P>SOLUTION: In an X-ray CT system, an annular (sector) phantom 330 uniform in thickness and cross section is positioned and scanned from many directions to acquire a plurality of views and the results of the scan is used to calculate a correction coefficient that is used to correct projection information to be acquired from a subject. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an X-ray CT apparatus that corrects (calibrates) the transmission intensity of a subject based on phantom data.

An X-ray CT apparatus is exemplified as the CT apparatus.
An X-ray source used in the X-ray CT apparatus outputs X-rays having a certain energy width. The X-ray absorption coefficient of X-rays transmitted through the subject depends on the X-ray energy, and exhibits a beam hardening (BH) effect in which the longer the transmission length of the subject, the higher the average energy moves. . Therefore, the transmission intensity of X-rays, that is, the projection information value generated from the signal detected by the X-ray detector in the X-ray CT apparatus, and the transmission length do not hold a proportional relationship, and have a non-linear relationship.

  The BH effect causes a cupping effect in which the intensity of the central portion is lowered on the reconstructed image in the X-ray CT apparatus. Therefore, it is necessary to correct the detection signal of the X-ray detector, and the reconstructed image having a uniform intensity. The correction is performed by obtaining the correction coefficient of the projection information value for generating X for each channel of the X-ray detector.

  Further, correction using a phantom is performed in order to perform high-precision correction. As such a phantom, an image of a phantom having a diameter that substantially covers the entire FOV (imaging region) arranged at the imaging center and having a plurality of cross-sections with different diameters is circular and cylindrical, and is corrected from projection information of these phantoms. The correction of the number of cases is refined (see, for example, Patent Document 1).

JP-A-7-171145

  In the above method, it takes a long time to repeatedly capture a plurality of phantoms having a large cross section and different diameters, which takes time to arrange the projection information when acquiring projection information, and the projection information value is caused by the beam hardening effect described above. It was not possible to perform high-accuracy correction in consideration of non-linear effects.

In addition, in order to correct projection information values with high accuracy, a large number of projection information values having different sizes are required for each channel, so that they are arranged at the center of the imaging region between the X-ray tube and the X-ray detector. It was necessary to image phantoms of various diameters having a circular cross section.
In particular, in the acquisition of calibration information of an X-ray CT apparatus, only two or three phantoms having a diameter of 20 cm to 50 cm and a circular section having a circular cross section are used, and the above correction is performed only for the precision of the correction. This requires time and effort for the calibration process.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an X-ray CT apparatus capable of acquiring calibration information, which can easily correct a beam hardening (BH) effect for each channel in consideration of nonlinear effects.

  According to the present invention, a phantom having an annular cross section and a substantially uniform thickness is positioned in an imaging region between an X-ray tube and an X-ray detector, and a plurality of views are acquired from one direction or multiple directions. Using the obtained projection information, correction coefficient calculation means for calculating a correction coefficient for correcting the imaging information of the subject using the imaging result, storage means for storing the correction coefficient, and correction of the imaging information of the subject There is provided an X-ray CT apparatus comprising correction means for correcting imaging information of the subject using a correction coefficient for performing the correction.

  Preferably, the correction coefficient calculation means is obtained by imaging a plurality of views from multiple directions by positioning a phantom having an annular cross section and a substantially uniform thickness in an imaging region between the X-ray tube and the X-ray detector. A first fitting unit that performs a process for smoothing variations in projection information, and a projection information value before processing by the first fitting unit and projection information after processing by the first fitting unit. Second fitting means for calculating a correction coefficient for correcting the imaging information of the subject by calculating a function related to the value.

  Preferably, the process for smoothing variations in the projection information is a smoothing process in the channel direction and the view direction.

  Preferably, the high-order fitting means uses the projection information value S (j) before processing by the first fitting means as the horizontal axis and the projection information value F (j) after processing by the first fitting means as the vertical axis. A second fitting is performed by plotting the graph as an axis.

  Preferably, the projection information value before processing by the first fitting means is a projection information value after beam hardening effect correction.

  Preferably, an X-ray CT apparatus is provided in which the first fitting means performs processing on a plurality of types of the annular phantoms having a substantially uniform thickness.

  Preferably, the plurality of types of the annular phantoms having substantially uniform thicknesses are processed with respect to each other in thickness, size, or orientation.

  Preferably, the correction coefficient calculating means calculates a quadratic or higher order function related to the projection information value before processing by the first fitting means and the projection information value after processing by the first fitting means. High-order fitting means for obtaining a correction coefficient for correcting the imaging information of the specimen is further provided.

Preferably, the first fitting means positions a phantom having a circular cross section at a plurality of positions shifted from the center position of the bore in an imaging region between the X-ray tube and the X-ray detector, and displays a plurality of views from multiple directions. Means for smoothing variations in projection information for the projection information obtained by photographing the first information, and the second fitting means includes the first fitting information in the projection information based on the phantom having a circular cross section. Means for calculating a function relating to a projection information value before processing by the fitting means and a projection information value after processing by the first fitting means to obtain a correction coefficient for correcting imaging information of the subject;
The correction coefficient calculation means calculates a correction coefficient for finally correcting the imaging information of the subject by using a correction coefficient based on a phantom having a circular cross section and a correction coefficient based on a phantom having an annular cross section and a substantially uniform thickness. Comprehensive correction coefficient calculating means for calculating is further provided.

  Preferably, the calculation of the total correction coefficient is based on a correction coefficient for finally correcting the imaging information of the subject, a correction coefficient based on a circular phantom of the cross section and a phantom having an annular cross section and a substantially uniform thickness. Calculated as an average value with the correction coefficient.

According to the present invention, it is possible to obtain a correction coefficient of imaging information suitable for the shape and part of a subject. If the imaging information of the subject is corrected using such a correction coefficient, a more accurate tomographic image can be obtained.
Further, according to the present invention, correction coefficients applicable to various subjects can be obtained. If the imaging information of the subject is corrected using such a correction coefficient, more accurate tomographic images can be obtained for various subjects.

Exemplary embodiments of a correction coefficient calculation method, a beam hardening post-processing method in a CT apparatus according to the present invention, and a CT apparatus using them will be described below with reference to the accompanying drawings.
In the present embodiment, an X-ray CT apparatus using X-rays as irradiation rays is exemplified as the CT apparatus.

Configuration of X-ray CT Apparatus The overall configuration of the X-ray CT apparatus according to this embodiment will be described with reference to FIG. The X-ray CT apparatus illustrated in FIG. 1 is applied to all the embodiments in this specification.
The X-ray CT apparatus illustrated in FIG. 1 includes a scanning gantry 2, an imaging table 4, and an operation console 6.

The scanning gantry scanning gantry 2 includes a rotating unit 34 and a rotation controller 36 that rotates the rotating unit 34.
As illustrated in FIG. 3 by enlarging the cross section, the rotating unit 34 is provided with the X-ray tube 20 and the X-ray detector 24 arranged to face each other via the bore 29. Further, the voile filter 21 (not shown in FIG. 1), a collimator 22, a collimator controller 30, an X-ray controller 28, and a data collection unit 26 are mounted on the rotation unit 34.
At the time of examination, the subject is positioned in the bore 29, and at the time of calibration, the bore 29 has the phantom 310 having a circular cross section illustrated in FIG. The subject or phantom located in the bore 29 is placed on a cradle in the bore 29 located at the center of the rotating unit 34.

  The rotating unit 34 rotates while being controlled by the rotation controller 36. In this rotation, X-rays are emitted from the X-ray tube 20 toward the X-ray detector 24, the transmitted X-rays of the subject and the phantom are detected by the X-ray detector 24, and the detection result of the X-ray detector 24 is obtained. Data collection unit 26 collects the data. The collection result is processed as projection information for each view in the operation console 6.

X-ray irradiation from the X-ray tube 20 is controlled by an X-ray controller 28. The X-rays radiated from the X-ray tube 20 are shaped by the collimator 22 into, for example, a fan-shaped X-ray beam, that is, a fan beam X-ray, and are further fan-shaped toward the X-ray detector 24 by the bow tile filter 21. The intensity diffused to the X-ray detector 24 is adjusted so as to be averaged over the entire surface of the X-ray detector 24, and enters the X-ray detector 24 through the bore 29.
The collimator 22 is controlled by a collimator controller 30.

As illustrated in FIG. 3, the X-ray detector 24 includes a plurality of X-ray detection elements arranged in an array in the spreading direction of the fan beam X-rays. As described above, the X-ray detector 24 is configured as a multi-channel detector in which a plurality of X-ray detection elements are arranged in an array, and as a whole, forms an X-ray incident surface curved in a cylindrical concave shape. To do. The X-ray detector 24 is configured by a combination of a scintillator and a photodiode, for example. The X-ray detector 24 may be, for example, a semiconductor X-ray detection element using cadmium tellurium (CdTe) or an ionization chamber type X-ray detection element using Xe gas.
The X-ray tube 20, the bow tile filter 21, the collimator 22 and the X-ray detector 24 constitute an X-ray irradiation / detection device according to the present invention.

  A data collection unit 26 is connected to the X-ray detector 24. The data collection unit 26 collects detection data of individual X-ray detection elements of the X-ray detector 24.

The operation console 6 includes a data processing device 60, a control interface 62, a data collection buffer 64, a storage device 66, a display device 68, and an operation device 70.
The data processing device 60 is constituted by a computer having a high data calculation processing function, for example. A control interface 62 is connected to the data processing device 60.
A scanning gantry 2 imaging table 4 is connected to the control interface 62. The data processing device 60 controls the scanning gantry 2 through the control interface 62. That is, the data collection unit 26, the X-ray controller 28, the collimator controller 30, and the rotation controller 36 in the scanning gantry 2 are controlled through the control interface 62 by the data processing device 60.

A data collection buffer 64 is connected to the data processing device 60. The data collection unit 64 of the scanning gantry 2 is connected to the data collection buffer 64. Data collected by the data collection unit 26 is input to the data processing device 60 through the data collection buffer 64.
The data processing device 60 performs image reconstruction using a transmission X-ray signal collected through the data collection buffer 64, that is, projection information. A storage device 66 is connected to the data processing device 60. The storage device 66 stores projection information collected in the data collection buffer 64, reconstructed tomographic image information, a program for realizing the functions of the X-ray CT apparatus of the present embodiment, and the like.

  A display device 68 and an operation device 70 are connected to the data processing device 60. The display device 68 displays tomographic image information and other information output from the data processing device 60. The operation device 70 is operated by an operator and inputs various instructions and information to the data processing device 60. The operator operates the X-ray CT apparatus of the present embodiment interactively (in an interactive manner) using the display device 68 and the operation device 70.

Shooting table 4
The imaging table 4 is connected to the data processing device 60 via the control interface 62, and displays various switches for operating the X-ray CT apparatus, operation tools, and X-ray CT images processed by the operation console 6. Equipment etc. are installed.

FIG. 2 shows a functional block diagram of only a portion related to the correction coefficient calculation method and the beam hardening post-processing method, which is the present embodiment of the data processing device 60.
When the data processing device 60 organizes the parts related to the present invention as means, the data collection means 201, the preprocessing means 202, the correction coefficient is calculated from the data preprocessed by the preprocessing means 202, and the storage device BH correction means 203, first fitting means 204, second fitting means 205, determination means 206, high-order fitting means 207, final correction processing means 208, which perform beam hardening correction processing performed on the projection information 66. And the image reconstruction means 209 is included.

The data collection unit 201 collects signals detected by the X-ray detector 24 with respect to the phantom via the data collection unit 26 and stores them in the storage device 66 as imaging information.
The pre-processing unit 202 performs processing before the beam hardening correction is performed on the imaging information, for example, noise removal processing.
The BH correction means 203 calculates correction coefficients B _{0 to} B ₃ for each channel, stores them in the storage device 66 as a correction coefficient table, and stores them in the storage device 66 using the correction coefficients B _{0 to} B ₃ . BH correction is performed on the projection information. When the projection information value acquired in each channel of the X-ray detector 24 is Ih and the correction data obtained by BH correction is IC, BH correction is performed by the following equation.

IC = B _O · Ih + B ₁ · Ih ² + B ₂ · Ih ³ + B ₃ · Ih ⁴ (1)

  The first fitting unit 204 smoothes data between each channel direction and each view of the projection information stored in the storage device 66. Since the high-frequency component exceeding the function order is removed from the function obtained by this fitting, the same effect as smoothing can be obtained.

  The second fitting means 205 is a linear or higher order function between the projection information value acquired by one channel of the X-ray detector 24 and the projection information value obtained by the first function fitting by the first fitting means 204. Perform fitting. Thereby, the correction coefficient similar to the equation (1) used in the BH correction unit 203 can be acquired.

The determination unit 206 determines whether or not to perform the above-described processing for different phantoms in order to improve the correction accuracy.
The high-order fitting means 207 obtains different phantoms and performs more advanced fitting using the correction coefficient.
The final correction processing unit 208 performs final correction on the photographing information using the correction coefficient obtained as described above.
The image reconstruction unit 209 reconstructs a tomographic image of a subject or a phantom, for example, a phantom 310 having a circular cross section illustrated in FIG. For the image reconstruction, for example, a filtered back projection method is used, and the reconstructed image is displayed on the display device 68.

First Embodiment As a first embodiment of the present invention, a case where a phantom 310 having a circular cross section is used in a bore 29 will be described. In particular, a case where the phantom 310 is disposed at a position shifted from the center position of the bore 29 will be described.
The phantom 310 is made of a material similar in composition to the human body that is the subject. For example, the phantom 310 has a cylindrical shape made of a material such as polypropylene and has a diameter of 35 cm, for example.
The basic operation of the X-ray CT apparatus for data collection, projection information and sinogram for the phantom 310 located in the bore 29 will be described.

FIG. 3 shows a phantom 310 having a circular cross section disposed in the bore 29 of the scanning gantry 2. The phantom 310 has a circular cross section, and its center is located at a location different from the imaging center of the bore 29.
The intensity of the X-rays emitted from the X-ray tube 20 is adjusted when transmitted through the bow tile filter 21 (smoothed in the channel direction of the X-ray detector 24), and the X-ray fan beam formed by the collimator 22 is The light passes through the phantom 310 having a circular cross section and is detected by the X-ray detector 24.

  The X-ray detector 24 has a plurality of X-ray detection elements arranged in an array in the direction in which the X-ray fan beam spreads, and detects projection information of the phantom 310 in each array channel.

FIG. 4 is a flowchart showing processing performed by each means of the data processing device 60.
Step 1: The phantom scan operator first places a phantom 310 having a circular cross section at a predetermined position in the bore 29. However, the phantom 310 is arranged at a position shifted from the imaging center in the bore 29. As described above, the phantom 310 having a circular cross section has a cylindrical shape made of a material such as polypropylene and has a diameter of, for example, 35 cm.
The data collection unit 201 of the data processing device 60 performs a phantom scan using a phantom 310 having a circular cross section. As a result, the first projection information 601 acquired by scanning is stored in the storage device 66. That is, the X-ray tube 20, the collimator 22 and the X-ray detector 24 are opposed to each other with the bore 29 as the center, and when rotating around the bore 29 together with the rotating unit 34 without changing the relative position, the data The data collection unit 201 of the data processing device 60 acquires the projection information via the collection unit 26 and stores it in the storage device 66.
As described above, the data collection unit 201 of the data processing device 60 acquires the projection information for each view number corresponding to the rotation angle, generates one sinogram, and stores it in the storage device 66.

  FIG. 5A is a diagram showing an example of a sinogram when a phantom 310 having a circular cross section is used. The sinogram is composed of a projection information portion existing near the center of the channel and an air data portion existing around the channel. Since the phantom 310 having a circular cross section is arranged so as to deviate from the imaging center, the position of the channel width of the projection information unit is changed with the rotation of the rotation unit 34, that is, the change of the view number. Meander in the view number direction as illustrated. For the same reason, the channel width of the projection information unit changes with the change of the view number.

  FIG. 5B is a diagram in which the projection information whose view number is j in FIG. 5A is displayed with the horizontal axis as the channel number and the vertical axis as the projection information value. Since the projection information value is proportional to the transmission length of the X-ray beam transmitted through the phantom 310 having a circular cross section, the X-ray transmitted through the vicinity of the center of the phantom 310 has a long transmission length and shows a high projection information value. The X-ray transmitted through the vicinity of the phantom 310 has a short transmission length and a low projection information value, and shows a semicircular projection image as shown in FIG.

As an example, a projection information value having a view number j and a channel number i is shown.
The X-ray beam indicated by the dotted line in FIG. 3 is incident on the channel number 1 of the X-ray detector 24 when the view number is j. At this time, the length of transmission of the X-ray beam through the phantom 310 having a circular cross section is assumed to be 1 (lower case L). This length l is proportional to the projection information value h of channel i in FIG. 5B as follows. That is, l∝h.
In FIG. 3, since the phantom 310 having a circular cross section is located at a position shifted from the imaging center, the transmission length l of the channel i changes for each view, and the channel i shown in FIG. The projection information value h also changes for each view.

  FIG. 5C shows the projection information value with the channel number i in FIG. 5A as the view number on the horizontal axis and the projection information value on the vertical axis. Since the projection information value is proportional to the transmission length of the X-ray beam that is different for each view number and passes through the phantom 310 having a circular cross section, the projection information value exhibits periodic characteristics as shown in FIG.

Step 2: Preprocessing FIG. 6 shows an intermediate projection information file stored in the storage device 66, which is generated during this operation.
The preprocessing means 202 of the data processing device 60 performs preprocessing such as noise removal and sensitivity correction on the sinogram composed of the first projection information.

Step 3: The BH correction means 203 of the beam hardening correction data processing device 60 performs BH correction on the projection information value Ih using the equation (1) to obtain a corrected projection information value Ic. The result is stored in the storage device 66 as the second projection information shown in FIG. In this file, the BH effect is substantially removed, but a slight BH effect due to the variation of each channel of the X-ray detector 24 remains. FIG. 7A schematically shows an example of the second projection information. Although it has a semicircular shape which is projection information of a substantially circular phantom, the projection information value Ic changes in a pulse shape depending on the X-ray sensitivity or the like depending on the channel. Since these are phenomena specific to a channel, it is necessary to correct each channel. FIG. 8A schematically illustrates an example of the projection information value in the view direction of one channel of the second projection information. Depending on the view number, the projection information value Ic changes in a pulse shape.

Step 4: Smoothing processing in the channel direction The first fitting means 204 of the data processing device 60 first performs smoothing in the channel direction using the second projection information 602. The result is stored in the storage device 66 as the 3A projection information 603 shown in FIG. In this projection information, the projection information value Ic resulting from the variation for each channel is smoothed and removed.
FIG. 7B schematically shows an example of the 3A projection information. Only the semicircular shape, which is the projection information of the circular phantom, is obtained as the projection information.

Step 5: The first fitting unit 204 of the smoothing data processing device 60 in the view direction further performs smoothing in the view direction using the projection information 603. Thereby, the 3B projection information 604 shown in FIG. 6 is generated. In this projection information, the projection information value resulting from the variation for each view occurring in one channel is smoothed. FIG. 8B schematically shows an example of the 3B projection information. A periodic projection information value in the view direction of one channel is smoothed.

Step 6: Calculation of primary correction coefficient The second fitting means 205 of the data processing device 60 obtains a primary correction coefficient from the second projection information and the third B projection information. Here, the projection information value of the second projection information with the channel number i is S (j), and the projection information value of the third B projection information is F (j).
Then, projection information values for all view numbers are arranged on a straight line that generally passes through the origin as shown in FIG. 9A when the horizontal axis is S (j) and the vertical axis is F (j). . Let this straight line be the correction coefficient for channel i.
FIG. 9A is a graph showing the relationship between the projection information value S (j) of the second projection information and the projection information value F (j) of the 3B projection information when there is one phantom.
This correction coefficient is stored in the storage device 66 as correction coefficient information 605. If the inclination of the correction coefficient is Ki, it can be expressed by the following equation.

                F (j) / S (j) ≈Ki

  When the projection information value Ic of the channel i after BH correction obtained from the subject is multiplied by this correction coefficient Ki, the following is obtained.

                Ip = Ic · Ki

As a result, a corrected projection information value Ip that has been smoothed is obtained.
The magnitude of the projection information value S (j) and the value acquisition range shown in FIG. 9A are the X-rays transmitted through the phantom 310 having the circular cross section shown in FIG. Since it is proportional to the transmission length l (el), it depends on the diameter of the circular phantom 310 and the position of the phantom in the bore 29.

Step 7: Determination of whether or not accuracy is improved The determination means 206 of the data processing device 60 determines whether or not the accuracy of the correction coefficient is improved. In the case of improving the accuracy of the correction coefficient, the operator arranges the phantom having a circular cross section and a circular cross section having a different diameter at a different position from the imaging center in the bore 29, and restarts the processing in steps 1 to 6. A new correction coefficient is acquired for the new phantom.

  FIG. 9B shows an example of correction coefficients obtained when a phantom having a circular cross section having two different diameters is used. The transmission length of the X-ray beam is determined from the diameter of the phantom and the position of the phantom, and the projection information value S (j) is also determined. Accordingly, assuming that the diameter of the phantom having a circular cross section is A and B, and A <B, the projection information value of the phantom A exists in the area A as shown in FIG. The information value exists in region B. Then, the correction coefficient can be obtained by the correction means 203 from the projection information values of these areas.

Step 8: Higher-order fitting When the above-described processing is performed using a plurality of phantoms and sufficient data is acquired for the accuracy of the correction coefficient, the higher-order fitting means 207 of the data processing device 60 is acquired. High-order function fitting is performed on the correction coefficient for each region. FIG. 10 shows an example in which the correction coefficients of phantoms A and B shown in FIG. 9B are used. The high-order fitting means 207 fits, for example, the following third-order fitting function to the value of the correction coefficient A in the area A and the value of the correction coefficient B in the area B, and determines the correction coefficients K0, Kl, and K2. To do.

If = K0 · S (j) + K1 · S (j) ² + K2 · S (j) ³ (2)

  At this time, since it is considered that the correction coefficient of the region A having a small projection information value is higher in accuracy than the correction coefficient of the region B having a large projection information value, the high-order fitting means 207 performs weighting for each region, The correction coefficient of equation (2) can also be determined so as to perform fitting with higher accuracy.

Step 9: Save correction coefficient The high-order fitting means 207 of the data processing device 60 saves the high-order correction coefficient information 606 composed of the values of the correction coefficients K0, Kl, and K2 in the storage device 66, and ends the processing.

Step 10: When the display subject is imaged, the final correction processing means 208 of the data processing device 60 applies the correction coefficient K0 for each channel to the projection information value Ic on which the BH correction of the subject has been performed. , Kl and K2, the corrected projection information value If is obtained from the equation (2).
These projection information values If are reconstructed by the image reconstruction means 209 to acquire tomographic image information, and are displayed on the display device 68 and / or the display section of the imaging table 4.

  As described above, in the first embodiment, a phantom having a circular section with a different diameter is arranged at a position shifted from the imaging center, and the transmission length of the X-ray beam differs from view to view. Since the projection information that is different for each view is acquired for each channel, the correction coefficient is approximated by a high-order function in the correction for each channel of the projection information value performed after the BH correction, and the nonlinear component is also taken into consideration. Correction can be performed, and even with a small number of phantom data, a highly accurate correction coefficient can be obtained. Therefore, the time and physical burden on the calibration process of the operator can be reduced.

  In the above-described method, fitting is performed with a cubic function using equation (2), but fitting can be performed with a quadratic function or a higher-order function of the fourth or higher order.

  In the present embodiment, the first-order correction coefficient is obtained for each sinogram as described with reference to step 6 in FIG. 4, but the second projection information 602 and the third projection information 604 for each sinogram. It is also possible to obtain a higher-order correction coefficient without fitting a first-order correction coefficient by fitting a higher-order function to.

  According to the first embodiment of the present invention, the first projection information of one or more phantoms having a circular cross section having different diameters arranged at positions shifted from the imaging center of the imaging region is obtained for all views. To obtain one or a plurality of sinograms, the beam hardening correction means corrects the beam hardening effect on the first projection information to generate second projection information, and the first fitting means Thus, the second projection information is subjected to the first function fitting to generate the third projection information, and the second fitting means performs the second fitting of all the views in each channel constituting the second projection information. A correction coefficient is obtained by performing a second function fitting on the value of the third projection information with the value of the projection information as an independent variable, and is placed in the imaging region from the correction coefficient by the correction means. Since the projection information of the subject to be corrected is corrected, the value of the second projection information is different for each view or sinogram. Therefore, when the correction coefficient is obtained by function fitting, the value of the second projection information is wide. The correction coefficient can be fitted within a range to improve the accuracy of the correction coefficient to improve the image quality, or the correction coefficient can be easily obtained by obtaining a high-precision correction coefficient with a small number of phantoms. it can.

In the first embodiment using the phantom 310 having a circular evaluation section according to the first embodiment, as illustrated in FIG. 3, the transmission length of X-rays transmitted through the phantom 310 having a circular section varies depending on the direction. Therefore, the X-ray transmission intensity reaching the X-ray detector 24 is not uniform. For example, in FIG. 3, the transmission length l1 is shorter than the transmission length l2.
On the other hand, the torso and head of the human body are rounded and oval.
Of course, in the first embodiment as well, the X-rays emitted from the X-ray tube 20 do not become inaccurate even if the X-ray transmission distances passing through the phantom 310 having a circular cross section are different. Can be adjusted so that the intensity of the X-rays incident on the X-ray detector 24 is made uniform, but in order to increase the accuracy, it is necessary to perform calibration by exchanging the bow tile filter 21 having various shapes. .
As described in the first embodiment, in order to perform high-accuracy correction, a plurality of cylindrical phantoms having a diameter that covers the entire FOV (imaging area) arranged at the center of imaging and having different diameters are imaged. The correction coefficient correction was refined from the projection information of these phantoms.

  As described above, when the method of the first embodiment is performed, the working time and the labor of the operator increase. Therefore, it is important how to realize a further post-hardening post-processing method and an X-ray CT apparatus that can easily and accurately correct the BH effect for each channel in consideration of nonlinear effects. In the second and subsequent embodiments, a method for improving the above-described problems of the first embodiment will be described.

Second Embodiment As a second embodiment, as illustrated in FIG. 11, an phantom 320 having an elliptical cross section is arranged in the imaging region in the bore 29 between the X-ray tube 20 and the X-ray detector 24. A method for performing BH correction on the phantom 320 having an elliptical cross section and calculating a correction coefficient will be described. FIG. 11 corresponds to FIG. 3 of the first embodiment.
The phantom 320 having an elliptical cross section approximates the cross section of the human torso. The material of the phantom 320 is the same as that of the phantom 310 having a circular cross section described in the first embodiment.
The difference between the first embodiment and the second embodiment is that the phantom 310 having a circular cross section is replaced with a phantom 320 having an elliptical cross section.
In the second embodiment, the X-ray CT apparatus to be applied is the same as that illustrated in FIG. Therefore, it is omitted to describe the X-ray CT apparatus with reference to FIG.
Also in the second embodiment, the configuration of the data processing device 60 described with reference to FIG. 2 is the same as that of the first embodiment. That is, the data processing device 60 includes a data collection unit 201, a preprocessing unit 202, a BH correction unit 203, a first fitting unit 204, a second fitting unit 205, a determination unit 206, a high-order fitting unit 207, and a final correction process. Means 208 has operator image reconstruction means 209.

The processing performed by each means of the data processing device 60 illustrated in FIG. 2 is the same as that of the first embodiment with reference to FIG. 4 except that the calibration method phantom is replaced by the phantom 320 having an elliptical cross section. . The outline is described.

Step 1 : The operator positions an phantom 320 having an elliptical cross section at the center line between the X-ray tube 20 and the X-ray detector 24 in the same imaging center portion of the imaging region as the subject is positioned. The phantom 320 having an elliptical cross section is a range of X-rays diffusing in a fan shape from the X-ray tube 20 to the X-ray detector 24, and the phantom 320 having an elliptical cross section at the end of the X-ray detector 24. The X-ray detector 24 is arranged so that X-rays that do not pass through the X-ray are detected.
The data collection unit 201 of the data processing device 60 acquires a single sinogram by photographing the first projection information about the phantom 320 having an elliptical section in a plurality of views from multiple directions.
The characteristic (profile) of the photographing information calculated by the data collecting unit 201 is, of course, different from that illustrated in FIGS. 5A to 5C for the phantom 310 having a circular cross section, and for the phantom 320 having an elliptical cross section. Will be.

Step 2 : The preprocessing means 202 of the data processing device 60 performs preprocessing similar to that of the first embodiment on the first imaging information as necessary.

Step 3 : The BH correction means 203 of the data processing device 60 corrects the beam hardening effect to the preprocessed first projection information and generates the second projection information, as in the first embodiment. Although the values of the coefficients B _{0 to} B _{3 in} the equation (1) are naturally different from the values in the first embodiment, the correction equations are the same.

Steps 4 to 5 : The first fitting means 204 of the data processing device 60 performs the smoothing process in the channel direction and the view direction as in the first embodiment.

Step 6 : The second fitting means 205 of the data processing device 60 obtains a primary correction coefficient as in the first embodiment.

Step 7 : If necessary, the operator, as illustrated in FIGS. 12A to 12B, for a phantom having a plurality of elliptical cross sections having different ellipticities or the like, or FIGS. 13A to 13C As illustrated in the above, whether or not to repeat the above processing is instructed for a phantom having a plurality of cross-sections having different dimensions, shapes and materials. The determination unit 206 determines whether there is such a request.
As such an phantom 320 having an elliptical cross section, a plurality of parts having a shape similar to a part to be particularly diagnosed by an X-ray CT apparatus and the body shape of the subject can be used.

Step 8 : After the above processing is completed for the phantom having a plurality of elliptical cross sections, the high-order fitting means 207 of the data processing device 60 fits a plurality of sets of correction coefficients to calculate a final correction coefficient.

Step 9 : The final correction processing means 208 corrects the photographing information using the final correction coefficient.
Step 10 : The image reconstructing means 209 of the data processing device 60 recalibrates the corrected image and displays it on the display device 68 or the like.

Since the phantom 320 having an elliptical cross section in the second embodiment is similar in shape to the torso, head, etc. of the human body as the subject, the phantom 310 having a circular cross section in the first embodiment is used. Thus, a correction coefficient more accurate than the correction coefficient obtained can be obtained.
Further, as illustrated in FIGS. 12A to 12B and FIGS. 13A to 13C, since correction coefficients are obtained for phantoms having various cross sections, subjects under various conditions, For example, it is possible to calculate a correction coefficient that can be applied in a unified manner for the head, chest, torso, leg, etc. of the same subject for adults or children, women or men, obese body skinny type, and the same subject.

Modified Embodiment In the above-described embodiment , the case has been described in which the various phantoms are repeatedly processed, and based on the results, the high-order fitting means 207 performs high-order fitting in step 8, but the cross-section is An individual correction coefficient can be calculated according to the shape and material of the elliptical phantom and stored in the storage device 66.
In the X-ray CT apparatus, the X-ray CT apparatus may be used for correction of imaging information according to the position of the subject, for example, the head, chest, torso, and leg. Similarly, a correction coefficient can be selected and used for the chest, torso, etc., depending on the conditions of the subject, such as a large subject or an obese subject. Further, the correction coefficient can be used by selecting according to whether the subject is a child or an adult.

Third Embodiment As a third embodiment, as illustrated in FIG. 14, an annular (fan-shaped) phantom 330 having a uniform cross section is formed between the X-ray tube 20 and the X-ray detector 24. A method of calculating the correction coefficient by arranging the phantom 330 in the imaging region in the bore 29 and performing BH correction on the phantom 330 will be described. FIG. 14 corresponds to FIG. 3 of the first embodiment.
The material of the annular (fan-shaped) phantom 330 having a uniform cross section is the same as that of the phantom 310 having a circular cross section described in the first embodiment.
The difference between the first embodiment and the second embodiment is that the phantom 310 having a circular cross section is replaced with an annular (fan-shaped) phantom 330 having a uniform cross section.
In the third embodiment, the X-ray CT apparatus to be applied is the same as that illustrated in FIG. Therefore, it is omitted to describe the X-ray CT apparatus with reference to FIG.
Also in the third embodiment, the configuration of the data processing device 60 described with reference to FIG. 2 is the same as that of the first embodiment. That is, the data processing device 60 includes a data collection unit 201, a preprocessing unit 202, a BH correction unit 203, a first fitting unit 204, a second fitting unit 205, a determination unit 206, a high-order fitting unit 207, and a final correction process. Means 208 has operator image reconstruction means 209.

2. Calibration Method Except that the phantom 310 having a circular cross section is replaced with the phantom 330 having an elliptical cross section, the processing performed by each means of the data processing device 60 illustrated in FIG. 2 is the first embodiment with reference to FIG. It is the same as the form. The outline is the same as that described in the second embodiment.

Step 1 : An operator has an annular (fan-shaped) phantom 330 whose section is uniform in the center line between the X-ray tube 20 and the X-ray detector 24 at the imaging center portion of the same imaging area where the subject is located. Position. Note that the phantom 330 is a range of X-rays diffusing in a fan shape from the X-ray tube 20 to the X-ray detector 24, and X-rays that do not pass through the phantom 330 at the end of the X-ray detector 24 are also X-rays. It arrange | positions so that it may detect with the detector 24. FIG.
The data collection unit 201 of the data processing device 60 acquires a single sinogram by photographing the first projection information about the phantom 330 in multiple views from multiple directions.
The characteristics (profile) of the photographing information calculated by the data collecting unit 201 for the annular (fan-shaped) phantom 330 having a uniform cross-section are, of course, FIGS. 5A to 5C for the phantom 310 having a circular cross-section. Unlike the example illustrated in (1), the cross-sectional (fan-shaped) phantom 330 has a uniform thickness.

Step 2 : The preprocessing means 202 of the data processing device 60 performs preprocessing similar to that of the first embodiment on the first imaging information as necessary.

Step 3 : The BH correction means 203 of the data processing device 60 corrects the beam hardening effect to the preprocessed first projection information and generates the second projection information, as in the first embodiment. Although the values of the coefficients B _{0 to} B _{3 in} the equation (1) are naturally different from the values in the first embodiment, the correction equations are the same.

Steps 4 to 5 : The first fitting means 204 of the data processing device 60 performs the smoothing process in the channel direction and the view direction as in the first embodiment.

Step 6 : The second fitting means 205 of the data processing device 60 obtains a primary correction coefficient as in the first embodiment.

Step 7 : If necessary, the operator may use an annular (fan-shaped) phantom having a plurality of different cross-sections having different thicknesses, dimensions, orientations, etc., as illustrated in FIGS. 15A to 15C. Instructs whether to repeat the above process. The determination unit 206 determines whether there is such a request.

Step 8 : After completing the above processing for the annular (fan-shaped) phantom 330 having a uniform thickness in a plurality of cross-sections, the high-order fitting means 207 of the data processing device 60 fits a plurality of sets of correction coefficients, and finally A typical correction factor is calculated.

Step 9 : The final correction processing means 208 corrects the photographing information using the final correction coefficient.
Step 10 : The image reconstructing means 209 of the data processing device 60 recalibrates the corrected image and displays it on the display device 68 or the like.

  According to the third embodiment, an accurate correction coefficient can be obtained even for the annular (fan-shaped) phantom 330 having a uniform cross section in the second embodiment.

Fourth Embodiment In the fourth embodiment, the correction coefficient obtained for various phantoms 310 having a circular cross section in step 9 in the first embodiment and the elliptical cross section in step 9 in the second embodiment. In order to obtain these common overall correction coefficients from the correction coefficients obtained for the various phantoms 320, the overall correction coefficient calculation means 210 is added to the data processing device 60 as illustrated in FIG.
As a method for calculating a total correction coefficient in the total correction coefficient calculation unit 210, for example, a correction coefficient obtained for various phantoms 310 having a circular cross section and a correction coefficient obtained for various phantoms 320 having an elliptical cross section. An average value can be taken. Alternatively, the sum can be obtained after multiplying by a predetermined weighting coefficient.
The total correction coefficient calculated in this way by the total correction coefficient calculation unit 210 is stored in the storage device 66 and can be used for correction of photographing information in the final correction processing unit 208.

  According to the fourth embodiment, a correction coefficient that can be widely applied to subjects under various conditions can be obtained, and using this correction coefficient enables accurate correction of imaging information.

Fifth Embodiment Unlike the fourth embodiment, the fifth embodiment differs from the fourth embodiment in the correction coefficients obtained for various phantoms 310 having a circular cross section in step 9 in the first embodiment, and the third embodiment. From the correction coefficient obtained for the annular (fan-shaped) phantom 330 having a uniform cross-section in step 9 in the embodiment, the total correction coefficient common to these common correction coefficients is calculated by the total correction coefficient calculation means 210 illustrated in FIG. Ask for.
As a method for calculating a total correction coefficient in the total correction coefficient calculation unit 210, for example, a correction coefficient obtained for various phantoms 310 having a circular cross section, and an annular (fan-shaped) phantom 330 having a uniform cross section thickness. It is possible to take an average value with the correction coefficient obtained for. Alternatively, the sum can be obtained after multiplying by a predetermined weighting coefficient.
The total correction coefficient calculated in this way by the total correction coefficient calculation unit 210 is stored in the storage device 66 and can be used for correction of photographing information in the final correction processing unit 208.

  According to the fifth embodiment, it is possible to obtain a correction coefficient that can be widely applied to subjects under various conditions, and using this correction coefficient enables accurate correction of imaging information.

Sixth Embodiment Unlike the fourth and fifth embodiments, the sixth embodiment differs from the fourth and fifth embodiments in the correction coefficient obtained for various phantoms 320 having an elliptical cross section in step 9 in the first embodiment, and From the correction coefficient obtained for the annular (fan-shaped) phantom 330 having a uniform cross section in step 9 in the third embodiment, the total correction coefficient calculation means 210 illustrated in FIG. Find the correct correction factor.
As a method for calculating a total correction coefficient in the total correction coefficient calculation unit 210, for example, a correction coefficient obtained for the phantom 320 having an elliptical cross section and a ring-shaped (fan-shaped) phantom 330 having a uniform cross section are obtained. The average value with the correction coefficient can be taken. Alternatively, the sum can be obtained after multiplying by a predetermined weighting coefficient.
The total correction coefficient calculated in this way by the total correction coefficient calculation unit 210 is stored in the storage device 66 and can be used for correction of photographing information in the final correction processing unit 208.

  According to the sixth embodiment, it is possible to obtain a correction coefficient that can be widely applied to subjects under various conditions, and using this correction coefficient enables accurate correction of imaging information.

Seventh Embodiment In the seventh embodiment, the correction coefficient obtained for the phantom 310 having a circular cross section in step 9 in the first embodiment and the phantom having an elliptical cross section in step 9 in the second embodiment. From the 320 correction coefficient and the correction coefficient obtained for the annular (fan-shaped) phantom 330 having a uniform cross section in step 9 in the third embodiment, the total correction coefficient calculating means 210 illustrated in FIG. These common correction factors are obtained.
As a method for calculating a total correction coefficient in the total correction coefficient calculation unit 210, for example, a correction coefficient obtained for a phantom 310 having a circular cross section, a correction coefficient obtained for a phantom 320 having an elliptical cross section, and a thickness of a cross section. Can be averaged with the correction coefficient obtained for the annular (fan-shaped) phantom 330. Alternatively, the sum can be obtained after multiplying by a predetermined weighting coefficient.
The total correction coefficient calculated in this way by the total correction coefficient calculation unit 210 is stored in the storage device 66 and can be used for correction of photographing information in the final correction processing unit 208.

  According to the seventh embodiment, a correction coefficient that can be widely applied to subjects under various conditions can be obtained, and using this correction coefficient enables accurate correction of imaging information.

  In carrying out the present invention, the present invention is not limited to the above-described examples, and various modifications can be made.

FIG. 1 is a block diagram showing the overall configuration of an X-ray CT apparatus as an embodiment of the CT apparatus of the present invention. FIG. 2 is a block diagram of a data processing apparatus in the X-ray CT apparatus illustrated in FIG. FIG. 3 shows the positional relationship between the X-ray tube, the X-ray detector, and the circular phantom when the phantom having a circular cross section is used as the first embodiment in the X-ray CT apparatus illustrated in FIG. FIG. FIG. 4 is a flowchart showing the operation of the data processing apparatus as the first embodiment. 5A to 5C are diagrams showing sinograms and projection information values when a phantom with a circular cross section is used as the first embodiment. FIG. 6 is a block diagram showing files in the storage device illustrated in FIG. 1 as the first embodiment. FIGS. 7A and 7B are diagrams illustrating processing of the projection information value in the channel direction as the first embodiment. FIGS. 8A and 8B are views showing the processing in the view direction of the projection information value as the first embodiment. FIGS. 9A and 9B are diagrams showing the correction coefficient of the projection information value as the first embodiment. FIG. 10 is a diagram for obtaining the second fitting function of the projection information value as the first embodiment. FIG. 11 shows the positional relationship between the X-ray tube, the X-ray detector, and the circular phantom when the phantom having an elliptical cross section is used as the second embodiment in the X-ray CT apparatus illustrated in FIG. FIG. 12A to 12B are various cross-sectional views of a phantom having an elliptical cross section according to the second embodiment. 13A to 13C are various cross-sectional views of a phantom having an elliptical cross section according to the second embodiment. 14 shows an X-ray tube, an X-ray detector, and an X-ray detector when an annular (fan-shaped) phantom having a uniform cross section is used as the third embodiment of the X-ray CT apparatus illustrated in FIG. It is a figure which shows the positional relationship with a phantom whose cross section is circular. FIGS. 15A to 15C are various cross-sectional views of an annular (fan-shaped) phantom having a uniform cross-section in the third embodiment. FIG. 16 is a second block diagram of the data processing apparatus in the X-ray CT apparatus illustrated in FIG.

Explanation of symbols

2. Scanning gantry
20 .. X-ray tube, 21 .. Botile filter, 22 .. Collimator 24 .. X-ray detector, 26 .. Data collection unit,
28 ... X-ray controller 29 ... Bore
30 ... Collimator controller 34 ... Rotary part
36..Rotation controller 4 .... Shooting table 6 .... Operation console
60 .. Data processing device
201 .. Data collection means
202 .. Pre-processing means
203 .. BH correction means
204 .. First fitting means
205..Second fitting means
206 .. Judgment means
207 ... Higher order fitting means
208 .. Final correction processing means
209 .. Image reconstruction means
210 .. Comprehensive correction coefficient calculation means
62..Control interface, 64..Data collection buffer
66..Storage device 68..Display device 70..Operating device 310..Phantom with a circular cross section 320..Phantom with an elliptical cross section 330..Annular (fan-shaped) phantom with uniform cross section 601... First projection information 602... Second projection information 603... 3A projection information 604... 3B projection information 605 .. Correction coefficient information 606.

Claims (10)

Using projection information obtained by imaging a plurality of views from one direction or multiple directions by positioning a phantom having an annular cross section and a substantially uniform thickness in an imaging region between the X-ray tube and the X-ray detector. Correction coefficient calculating means for calculating a correction coefficient for correcting imaging information of the subject using the imaging result;
An X-ray CT apparatus comprising: storage means for storing the correction coefficient; and correction means for correcting the imaging information of the subject using a correction coefficient for correcting the imaging information of the subject.   The correction coefficient calculating means is a projection obtained by imaging a plurality of views from multiple directions by positioning a phantom having an annular cross section and a substantially uniform thickness in an imaging region between the X-ray tube and the X-ray detector. A first fitting unit that performs a process of smoothing variations in projection information, and a projection information value before processing by the first fitting unit and a projection information value after processing by the first fitting unit. The X-ray CT apparatus according to claim 1, further comprising: a second fitting unit that calculates a function and calculates a correction coefficient for correcting imaging information of the subject.   The X-ray CT apparatus according to claim 1, wherein the process for smoothing variations in the projection information is a smoothing process in a channel direction and a view direction.   The higher-order fitting means uses the projection information value S (j) before processing by the first fitting means as the horizontal axis and the projection information value F (j) after processing by the first fitting means as the vertical axis. The X-ray CT apparatus according to claim 1, wherein the second fitting is performed by plotting on a graph.   The X-ray CT apparatus according to claim 1, wherein the projection information value before processing by the first fitting unit is a projection information value after beam hardening effect correction.   The X-ray CT apparatus according to claim 1, wherein the first fitting unit performs processing on a plurality of types of the annular phantoms having a substantially uniform thickness.   The X-ray CT apparatus according to claim 6, wherein the plurality of types of the annular phantoms having substantially uniform thicknesses are processed in different thicknesses, dimensions, or directions.   The correction coefficient calculating unit calculates a quadratic or higher order function related to the projection information value before the processing by the first fitting unit and the projection information value after the processing by the first fitting unit, thereby imaging the subject. The X-ray CT apparatus according to claim 6, further comprising high-order fitting means for obtaining a correction coefficient for correcting information. The first fitting means images a plurality of views from multiple directions by positioning a phantom having a circular cross section at a plurality of positions shifted from the center position of the bore in an imaging region between the X-ray tube and the X-ray detector. The projection information obtained by further comprising means for performing a process of smoothing the variation in projection information,
The second fitting means has a function relating to a projection information value before processing by the first fitting means and a projection information value after processing by the first fitting means in projection information based on the phantom having a circular cross section. Means for calculating a correction coefficient for calculating and correcting imaging information of the subject;
The correction coefficient calculation means calculates a correction coefficient for finally correcting the imaging information of the subject by using a correction coefficient based on a phantom having a circular cross section and a correction coefficient based on a phantom having an annular cross section and a substantially uniform thickness. The X-ray CT apparatus according to claim 1, further comprising a total correction coefficient calculation unit for calculating.   The total correction coefficient calculation is a correction coefficient for finally correcting the imaging information of the subject, a correction coefficient based on a phantom having a circular cross section and a correction coefficient based on a phantom having an annular cross section and a substantially uniform thickness. The X-ray CT apparatus according to claim 9, wherein the X-ray CT apparatus is calculated as an average value.

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