JP5608001B2 - Radiographic imaging method and apparatus - Google Patents

Description

  The present invention acquires a radiographic image signal for each predetermined imaging angle by rotating the radiographic image detector around the subject, performs a reconstruction process based on the radiographic image signal for each imaging angle, and The present invention relates to a radiographic imaging method and apparatus for generating a tomographic image.

  Conventionally, a radiation source and a radiation image detector are arranged facing each other around a subject, and these sets are rotated around the subject, and radiation images are taken by irradiating radiation from various angles. A radiation CT image capturing system that reconstructs a tomographic image using an angle radiation image and displays an arbitrary cross section is widely used in clinical practice.

  Here, the reconstruction methods in the radiation CT imaging system described above can be broadly divided into analytical solutions (for example, FBP (Filtered Back Projection) method, etc.), and the line absorption coefficient of a fault is modeled as a matrix, and statistical And a method of solving using a general idea (for example, ML-EM (Maximum Likelihood-Expectation Maximization) method). The former is called an analytical method, and the latter is called an algebraic or statistical method. In the present specification, the latter is collectively referred to as a statistical method. Statistical methods include a successive approximation method and an iterative method that perform iterative calculations.

  The successive approximation method has an advantage that the S / N is generally higher than that of the FBP method and streak artifacts are not generated from the high-contrast part. There are drawbacks.

  Therefore, until now, it was rarely used in actual diagnostic equipment due to the problem of reconstruction time. However, due to its high S / N, it has recently attracted attention again.

Here, the noise characteristics of the FBP method and the successive approximation method will be described. Consider a case where the resolution of the radiation image detector is increased by n times. Since the detector element area is roughly proportional to the square of the element pitch, the element area becomes 1 / n ² when the resolution is increased by n times. In this case, when the image is taken with the same X-ray dose, the dose received by one element is also 1 / n ² .

The noise factors of the radiation image detector include X-ray quantum noise, structural noise, detection noise, aliasing noise, and electric noise. Of these, X-rays are dominant in many cases. It is quantum noise. X-ray quantum noise is caused by a probabilistic variation in the number of photons falling per unit area, and is obtained by the square root of the average value (dose) of the number of photons per unit area. This is because the photon existence probability follows a Poisson distribution. That is, when an average of N photons are irradiated per unit area, N ^1/2 is obtained when a variation (standard deviation) in the actual number of photons observed in each unit lattice is obtained. Therefore, S / N is N ÷ N ^1/2 = N ^1/2 .

Returning to the description of the case where the resolution becomes n times, the dose received by one element becomes 1 / n ² means that the S / N due to X-ray quantum noise becomes 1 / n. Become. This is the noise characteristic of a two-dimensional radiographic image detected by the radiographic image detector. Since the three-dimensional tomographic image is also affected by the two-dimensional projection image, the S / N is considered to be 1 / n when the resolution is increased n times, but this is not the case with the FBP method. That is, in the case of the FBP method, S / N is 1 / n ² . This is due to the reconstruction filter function used in the reconstruction of the FBP method. The reconstruction filter function has a property that the higher the frequency, the higher the noise component. As a result, the high frequency noise component is amplified, so that the S / N deteriorates as the resolution of the image increases.

  On the other hand, since the successive approximation method including the ML-EM method does not amplify the high frequency uniformly unlike the reconstruction filter function, the S / N is 1 as in the case of the two-dimensional radiation image. It only needs to be / n.

In order to keep the S / N constant even if the resolution is increased, it is sufficient to increase the irradiation dose and keep the dose per element constant. However, when the resolution becomes n times, the FBP method The dose of n ⁴ times is required, and in the case of the successive approximation method, the dose of n ² times is required. That is, the higher the resolution, the more advantageous the successive approximation method from the viewpoint of S / N.

  The resolution of X-ray CT apparatuses in recent years is about 250 μm for general use, 200 μm for angiography, and about 100 μm for dental use. However, if higher resolution is to be achieved, the FBP method will increase the exposure dose too much. For this reason, reconstruction methods using the successive approximation method have begun to be used.

Up to this point, the advantages of the successive approximation method have been described focusing on the relationship between the resolution and the S / N, but the successive approximation method has one more advantage over the FBP method with respect to the S / N. That is the relationship between the number of shots and S / N. For example, when the number of shots becomes n times, the S / N when using the FBP method becomes n ^1/2 times. This is the same as the concept of the average photon number and variation at the time of resolution. On the other hand, the successive approximation method has the property that the S / N hardly changes as the number of shots changes. That is, if the dose per radiation image is sufficient, the S / N does not deteriorate even if the number of sheets is reduced, and the exposure dose can be reduced by the amount of reduction.

  However, reducing the number of shots causes image quality deterioration at points other than S / N. Specifically, it does not converge to a true solution due to lack of data, and as shown in FIG. 9, streak artifacts are generated from the edge portion and the contrast of a minute object is reduced.

  This problem can be solved by giving foresight information to the solution. Conventionally, it is considered that there is no problem with a normal solid image (a plain image) as an initial image given to the successive approximation method.

  By giving the characteristics of the tomographic image known from the beginning to this initial image, the convergence to the solution can be increased and the convergence to the wrong solution can be prevented. Patent Document 1 proposes a method of setting an initial image of the successive approximation method, and Patent Document 1 proposes a method of giving a reconstructed image by the FBP method as an initial image.

JP 05-168620 A

  However, if the number of shots is simply reduced and a reconstructed image based on the FBP method is used as the initial image, the artifacts may be generated even if the FBP method is used in a state where the number of projections is insufficient to produce an artifact by the successive approximation method. It will come out. When reconstructing using the successive approximation method, the streak artifacts generated by the FBP method may remain converged.

  In view of the above circumstances, an object of the present invention is to provide a radiographic imaging method and apparatus capable of acquiring a reconstructed image free from artifacts while taking advantage of the noise gain of the successive approximation method with imaging with a small dose. And

  The radiographic imaging method of the present invention is a radiographic image detector that detects radiation emitted from a radiation source and transmitted through the subject and outputs a radiographic image signal representing the radiographic image of the subject, and at least one of the radiation source and the subject A radiographic image signal for each radiographing angle output from the radiographic image detector by irradiating the subject with radiation for each predetermined radiographing angle, and based on the radiographic image signal for each radiographing angle In the radiographic imaging method for generating a tomographic image of a subject by performing reconstruction, the radiation source is controlled so that a first dose of radiation is emitted at some imaging angles, and at other imaging angles. The radiation source is controlled so that a second dose of radiation lower than the first dose is emitted, and the radiation is irradiated by the irradiation of the first dose of radiation. Based on the first radiation image signal output from the image detector and the second radiation image signal output from the radiation image detector by irradiation with the second dose of radiation, reconstruction is performed using an analytical method. To generate an initial reconstructed image, and based on the initial reconstructed image and the first radiation image signal, reconstruct using a statistical method to generate a tomographic image of the subject. And

  A radiation image capturing apparatus of the present invention includes a radiation source that emits radiation, a radiation image detector that detects radiation emitted from the radiation source and transmitted through the subject, and outputs a radiation image signal representing the radiation image of the subject; A rotation drive unit that circulates at least one of the radiation source and the radiation image detector around the subject, and radiation at each imaging angle output from the radiation image detector by irradiation of the subject with radiation at each predetermined imaging angle In a radiographic imaging apparatus including a reconstruction unit that acquires an image signal and reconstructs based on a radiographic image signal for each imaging angle to generate a tomographic image of the subject, the radiographic imaging apparatus includes a first at some imaging angles. The radiation source is controlled such that a second dose of radiation is emitted, and a second dose of radiation lower than the first dose is emitted at other imaging angles. A radiation source control unit that controls the radiation source, and the reconstruction unit includes a first radiation image signal output from the radiation image detector by irradiation of the first dose of radiation and a second dose of radiation. Based on the second radiation image signal output from the radiation image detector by irradiation of radiation, reconstruction is performed using an analytical method to generate an initial reconstructed image, and the initial reconstructed image and the first reconstructed image The tomographic image of the subject is generated by performing reconstruction using a statistical method based on the radiographic image signal.

  In the radiographic imaging apparatus of the present invention, the reconstruction unit can use the FBP method as an analytical method.

  Further, the reconstruction unit can use a successive approximation method as a statistical method.

  In addition, it is possible to provide a high-frequency removal unit that performs a process of removing a high-frequency component on the initial reconstructed image or the first radiation image signal and the second radiation image signal. In addition, a reduction processing unit that performs reduction processing on the first radiation image signal and the second radiation image signal can be provided.

  In addition, an enlargement processing unit that performs an enlargement process on the initial reconstructed image generated based on the first radiographic image signal and the second radiographic image signal subjected to the reduction process can be provided.

  Further, the radiological image detector is composed of a large number of pixels, and when the second radiographic image signal is read out from the radiographic image detector, a reading control unit that performs binning reading that adds and reads out the charge signals of a plurality of pixels And a reduction processing unit that performs a reduction process on the first radiation image signal.

  Further, enlargement processing is performed on the initial reconstructed image generated based on the first radiation image signal subjected to the reduction processing and the second radiation image signal output from the radiation image detector by binning readout. An enlargement processing unit to be applied can be provided.

  A binning process can be used as the reduction process.

  The radiation source control unit controls the radiation source so that the number of imaging angles at which the first dose of radiation is emitted is smaller than the number of imaging angles at which the second dose of radiation is emitted. I can do it.

  Further, the number of imaging angles at which the first dose of radiation is emitted can be 1/5 or more and 1/3 or less of the imaging angle at which the second dose of radiation is emitted.

  Further, the second dose can be set to 1/8 or more and 1/4 or less of the first dose.

  According to the radiographic imaging method and apparatus of the present invention, the radiation source is controlled so that the first dose of radiation is emitted at a part of the imaging angles, and at the other imaging angles than the first dose. The radiation source is controlled so that a lower second dose of radiation is emitted, and the first radiation image signal and the second dose of radiation output from the radiation image detector by the irradiation of the first dose of radiation. Based on the second radiographic image signal output from the radiographic image detector by the irradiation, an reconstruction is performed using an analytical method to generate an initial reconstructed image. The initial reconstructed image and the first reconstructed image Since the tomographic image of the subject is generated by using a statistical method based on the radiographic image signal, the number of captured images is not reduced for the initial reconstructed image. The generation of artifacts can be suppressed, and some radiation image signals can be taken at a low dose, so that the exposure dose can be reduced, and the final reconstruction can be performed statistically. By using this method, a tomographic image with a good S / N can be acquired.

  In addition, when processing for removing high-frequency components is performed on the initial reconstructed image or the first radiation image signal and the second radiation image signal, the radiation image signal of low-dose imaging is used. S / N degradation that occurs can be suppressed, and a tomographic image with improved S / N can be acquired.

1 is a schematic configuration diagram of a radiation CT image capturing system using an embodiment of a radiation image capturing apparatus of the present invention. The block diagram which shows the internal structure of a radiation detection part and a computer in the radiation CT imaging system using one Embodiment of the radiographic imaging apparatus of this invention. The flowchart for demonstrating the effect | action of imaging | photography of the radiation CT image imaging system using one Embodiment of the radiographic imaging apparatus of this invention. The schematic diagram which shows the relationship between the imaging angle and the radiation dose in the radiation CT image imaging system using one Embodiment of the radiographic imaging apparatus of this invention. The schematic diagram which shows the relationship between the imaging angle and radiation dose of other embodiment of the radiographic imaging apparatus of this invention. The flowchart for demonstrating the effect | action of the reconstruction of the radiation CT image imaging system using one Embodiment of the radiographic imaging apparatus of this invention. The block diagram which shows the internal structure of a radiation detection part and a computer in the radiation CT imaging system using other embodiment of the radiographic imaging apparatus of this invention. The block diagram which shows the internal structure of a radiation detection part and a computer in the radiation CT imaging system using other embodiment of the radiographic imaging apparatus of this invention. Illustration for explaining streak artifacts

  A radiation CT image capturing system using an embodiment of a radiation image capturing apparatus of the present invention will be described below with reference to the drawings. First, a schematic configuration of the entire radiation CT image capturing system will be described. FIG. 1 is a diagram showing a schematic configuration of the present radiation CT image capturing system.

  As shown in FIG. 1, the radiation CT image imaging system is connected to an imaging apparatus 1 that captures a radiographic image of a subject P, a bed 22 that is a support base for supporting the subject P, and the imaging apparatus 1. The computer 30 controls the imaging apparatus 1 and processes a radiation image signal obtained by imaging, and a monitor 31 connected to the computer 30.

  The imaging apparatus 1 includes a radiation source 10 that emits conical radiation, a radiation detection unit 11 that detects radiation emitted from the radiation source 10, a radiation source 10, and a radiation detection unit 11 that face each end. A C-arm 12 that holds them, a rotation drive unit 15 that rotates the C-arm 12, and an arm 20 that holds the rotation drive unit 15.

  The C arm 12 is attached to the rotation drive unit 15 so as to be able to rotate 360 ° around the rotation axis C. The arm 20 includes a movable portion 20a and is held by a base portion 21 that is movably installed with respect to the ceiling. The C-arm 12 can be moved to a wide range of positions in the photographing room by moving the base 21, and the rotation direction (rotation axis angle) can be changed by moving the movable part 20a of the arm 20. Has been.

  The radiation source 10 and the radiation detection unit 11 are disposed to face each other with the rotation axis C interposed therebetween. When performing radiation CT image capturing, the positional relationship between the rotation axis C, the radiation source 10 and the radiation detection unit 11 is as follows. In a fixed state, the C arm 12 is rotated 180 ° to 360 ° by the rotation driving unit 15.

  FIG. 2 is a block diagram illustrating a schematic configuration inside the radiation detection unit 11 and the computer 30.

  As shown in FIG. 2, the radiation detection unit 11 receives a radiation that has passed through the subject P, generates charges, and outputs a radiation image signal representing a radiation image of the subject P. And a signal processing unit 11b that performs predetermined signal processing on the radiation image signal output from the image detector 11a.

  The radiological image detector 11a can repeatedly perform recording and reading of radiographic images, and a so-called direct type radiographic image detector that directly receives radiation and generates charges may be used. Alternatively, a so-called indirect radiation image detector that converts radiation once into visible light and converts the visible light into a charge signal may be used. Further, as a radiation image signal readout method, it is desirable to use a so-called TFT readout method in which a radiation image signal is read out by turning on and off a TFT (thin film transistor) switch. You may make it use not only other things. In the present embodiment, it is assumed that a TFT readout type radiation image detector having a pixel size of 200 μm is used.

  The signal processing unit 11b includes an amplifier unit including a charge amplifier that converts the charge signal read from the radiation image detector 11a into a voltage signal, and an AD conversion unit that converts the voltage signal output from the amplifier unit into a digital signal. Etc.

  The computer 30 includes a central processing unit (CPU) and a storage device such as a semiconductor memory, a hard disk, and an SSD, and the low-dose image memory 30a, the high-dose image memory 30b, the LPF processing unit 30c, An FBP method reconstruction unit 30d, an initial reconstruction image memory 30e, a successive approximation method reconstruction unit 30f, and an imaging control unit 30g are configured.

  The low-dose image memory 30a temporarily stores the second radiation image signal detected by the radiation image detector 11a by irradiating the subject with a relatively low dose of radiation at a part of the imaging angle of the C-arm 12. It is something to remember.

  The high-dose image memory 30b is the first detected by the radiation image detector 11a by irradiating the subject with a relatively high dose of radiation at an imaging angle of the C-arm 12 other than the imaging angle of the low-dose radiation image described above. One radiation image signal is temporarily stored.

  The LPF processing unit 30c is a low-pass that removes a high-frequency component from the second radiation image signal read from the low-dose image memory 30a and the first radiation image signal read from the high-dose image memory 30b. Filter processing is performed.

  The FPB method reconstruction unit 30d uses the first radiation image signal and the second radiation image signal that have been subjected to the low-pass filter processing in the LPF processing unit 30c, and performs reconstruction using an analytical technique to perform initial processing. A reconstructed image is generated. Specifically, in the present embodiment, an initial reconstructed image is generated using an FBP (Filtered Back Projection) method as a reconstruction method of the analytical technique. In the embodiment, the FBP method is used, but other reconstruction methods may be used as long as they are analytical methods. For example, an FFT (Fast Fourier Transform) method or a convolution method is used. be able to.

  The initial reconstructed image memory 30e temporarily stores the initial reconstructed image generated by the FBP method reconstructing unit 30d.

  The successive approximation method reconstruction unit 30f uses a statistical method using the initial reconstructed image read from the initial reconstructed image memory 30e and the first radiation image signal read from the high-dose image memory 30b. Is used to generate a final tomographic image of the subject. Specifically, in this embodiment, a tomographic image is generated using a successive approximation method that obtains a convergent solution by iterative calculation as a reconstruction method of a statistical method. In this embodiment, the ML-EM (Maximum Likelihood-Expectation Maximization) method is used as one of the successive approximation methods. However, the present invention is not limited to this. A Posteriori-Expectation Maximization (ART) method, other methods classified as ART (Algebraic Reconstruction Techniques), SIRT method (Simultaneous Interactive Reconstruction Techniques), SART (Statistical Algebraic Reconstruction Techniques), IRT (Iterative Reconstruction Techniques) it can.

  The imaging control unit 30g drives and controls the rotation operation of the C arm 12 by the rotation driving unit 15 and the irradiation timing of the radiation emitted from the radiation source 10. A specific control method will be described in detail later.

  The monitor 31 displays a tomographic image or a three-dimensional image composed of a plurality of tomographic images based on an image signal representing a tomographic image of a subject output from the computer 30.

  Next, the operation of this radiation CT image capturing system will be described with reference to the flowchart shown in FIG.

  First, the subject P is laid on the bed 22, and the radiation source 10 and the radiation detection unit 11 are arranged at symmetrical positions with the rotational axis C being the approximate center of the subject P's body. Thus, positioning of the C arm 12 is performed. The movement of the C-arm 12 is performed based on the operation of the computer 30 by the user.

  Next, photographing conditions are input by the photographer using a predetermined input unit. Specifically, the radiation dose, the number of radiographs, and the imaging angle pitch when capturing a radiographic image by irradiating with a high dose of radiation, and the radiation dose when capturing a radiographic image by irradiating with a low dose of radiation The number of images and the shooting angle pitch are input. Specifically, in this embodiment, as a high-dose imaging condition, a dose of 20 uGy / projection per radiographic image, 50 imaging images, and an imaging angle pitch of 4 degrees are set and input. Assuming that the dose per radiation image is 2.5 uGy / projection and the number of radiographs is 150, and the radiographing angle pitch is set and inputted so that radiographing is performed at intervals of 1 degree.

  Here, in the present embodiment, the number of shots and the radiation dose are set as described above. The effect of reducing the exposure dose at this time will be described below.

  First, considering the exposure dose in conventional imaging methods, the FBP method is used for the number of images to make streak artifacts inconspicuous from high-contrast areas such as calcification and body surface, depending on the size of the subject and the contrast of the subject. In the case of the ML-EM method, the number is about 100. Further, in the ML-EM method, the conditions for preventing the contrast of calcification from being lowered are approximately this level. Therefore, using the ML-EM method, if the radiation dose per radiographic image is 20 uGy / projection and the number of radiographs is 100, the total exposure dose is 2 mGy.

  On the other hand, according to the imaging conditions of this embodiment described above, the radiation dose per radiographic image is 20 uGy / projection, the number of radiographs is 50, and the radiographic image 1 is used as the low-dose imaging condition. Since the radiation dose per shot was 2.5 uGy / projection and the number of shots was 150, the total exposure was 1 mGy + 375 uGy = 1.38 mGy, which was about 30% of the conventional total exposure described above. It can be seen that it is reduced.

  In this embodiment, the number of high-dose shots is set to 1/3 of the number of low-dose shots. However, the number of high-dose shots is smaller than the number of low-dose shots. If the number is set, other number of shots may be set. However, it is more preferable that the number of high-dose imaging be 1/5 or more and 1/3 or less of the number of low-dose imaging.

  In the present embodiment, the dose for low-dose imaging is set to 1/8 of the dose for high-dose imaging, but other ratios may be set. However, it is more preferable that the dose for low-dose imaging is 1/8 to 1/4 of the dose for high-dose imaging.

  The imaging conditions set and input are input to the imaging control unit 30g, and the imaging control unit 30g creates a radiation dose table for each imaging angle based on the imaging conditions set and input (S12). In the present embodiment, as described above, shooting is performed with a high dose at a shooting angle pitch of 4 degrees, and the high dose shooting is set to be shot with a low dose in increments of 1 degree. Thus, a dose table is created in which high-dose imaging and low-dose imaging are alternately repeated. This dose table is a table in which an imaging angle is associated with a radiation dose at the imaging angle. FIG. 4 is a schematic diagram showing the shooting timing, and does not accurately represent the shooting angle.

  In the present embodiment, high-dose imaging and low-dose imaging are performed at the imaging timing as shown in FIG. 4, but not limited thereto, for example, as shown in FIG. High-dose imaging may be performed in the range of the imaging angle of 180 degrees, and low-dose imaging may be performed in the range of 180 degrees to 360 degrees.

  Then, a photographing start button (not shown) is pressed by the photographer, a photographing start instruction is input, and the rotation operation of the C-arm 12 is started (S12).

  Then, the imaging control unit 30g sequentially acquires imaging angles corresponding to the rotation of the C-arm 12 (S16), and when the imaging angle becomes the imaging angle set in the dose table (S20), the imaging The dose of the radiation at the angle is acquired from the dose table, and a control signal is output to the radiation source 10 so that the radiation corresponding to the dose is emitted.

  Then, radiation is emitted from the radiation source 10 in accordance with the control signal output from the imaging control unit 30g, and the radiation transmitted through the subject P is detected by the radiation image detector 11a and detected by the radiation image detector 11a. The read charge signal is read out.

  The charge signal read from the radiation image detector 11a is output to the computer 30 after being subjected to predetermined processing in the signal processing unit 11b. If the radiation image signal output to the computer 30 is acquired by low-dose imaging, it is stored in the low-dose image memory 30a of the computer 30 and acquired by high-dose imaging. In some cases, it is stored in the high-dose image memory 30b of the computer 30 (S24).

  Then, based on the imaging angle of the dose table and the radiation dose, the irradiation of radiation and the reading and storage of the radiation image signal from the radiation image detector 11a are sequentially performed in the same manner as described above (S16 to S24). When the angle reaches the photographing end angle, the rotation of the C-arm 12 is stopped and the photographing is finished (S18, YES, S26).

  Next, a high-dose radiographic image signal (hereinafter referred to as a first radiographic image signal) and a low-dose radiographic image signal (hereinafter referred to as a second radiographic image signal) stored in the computer 30 as described above. The operation for generating a tomographic image of the subject P based on the above will be described with reference to the flowchart shown in FIG.

  First, the second radiation image signal is read from the low-dose image memory 30a, and the first radiation image signal is read from the high-dose image memory 30b (S30). Then, the read first and second radiation image signals are input to the LPF processing unit 30c, and the LPF processing unit 30c performs low-pass filter processing on the first and second radiation image signals ( S32).

  The first and second radiation image signals subjected to the low-pass filter processing are input to the FBP method reconstruction unit 30d, and the FBP method reconstruction unit 30d performs the first radiation image signal and the second radiation image signal. Using the signal, reconstruction processing is performed by the FBP method to generate an initial reconstructed image (S34). The initial reconstructed image is stored in the initial reconstructed image memory 30e (S36).

  Next, the first radiation image signal stored in the high-dose image memory 30b and the initial reconstructed image stored in the initial reconstructed image memory 30e are read out and input to the successive approximation method reconstructing unit 30f. (S38). Then, in the successive approximation method reconstruction unit 30f, an initial reconstruction image is used as an initial value, and a tomographic image of the subject P is generated by performing reconstruction processing by the ML-EM method using the first radiation image signal. (S40, S42).

  Then, predetermined signal processing is performed on the image signal representing the tomographic image generated by the successive approximation method reconstruction unit 30 f to generate a display tomographic image signal, and the display tomographic image signal is output to the monitor 31. Then, a tomographic image of the subject P is displayed on the monitor 31 based on the display tomographic image signal. The image displayed on the monitor 31 may be a single tomographic image or a three-dimensional image composed of a plurality of tomographic images.

  In the description of the above embodiment, the low-pass filter processing is performed on the first and second radiation image signals before reconstructing the initial reconstructed image. On the other hand, processing for removing high frequency components may be performed. Specifically, a reconstruction filter function used in the reconstruction process by the FBP method may be used having a characteristic that lowers the high frequency component.

  In addition, as described above, the first and second radiographic image signals and the initial reconstructed image are not subjected to the process of removing the high frequency component, but the first and second radiographic image signals are reduced. The same effect can be obtained by applying the above. Here, a case where the binning process is performed as one of the reduction processes will be described. However, the present invention is not limited to the binning process, and other reduction processes may be performed.

  As shown in FIG. 7, the computer 35 according to the embodiment that performs the binning process described above, instead of the LPF processing unit, a binning processing unit 30 h that performs binning processing on the first and second radiation image signals, and an enlargement And a processing unit 30i.

  In the case of this embodiment, a binning processing unit is applied to the first radiation image signal read from the high-dose image memory 30b and the second radiation image signal read from the low-dose image memory 30a. A binning process is performed in 30h. The binning process is a process that, for example, averages a plurality of pixel signals into one pixel signal. As for the number of pixels added in the binning process, the number of pixels corresponding to a desired cutoff frequency is set in the binning processing unit 30h in advance.

  Then, the first and second radiographic image signals after the binning processing are input to the FBP method reconstructing unit 30d, and the FBP method reconstructing unit 30d receives the first radiographic image signal and the second radiographic image signal. Is used, the reconstruction process is performed by the FBP method, and an initial reconstructed image is generated.

  Next, the initial reconstructed image generated by the FBP method reconstruction unit 30d is output to the enlargement processing unit 30i, and the enlargement processing unit 30i performs enlargement processing. The reason why the enlargement process is performed in this way is that the resolution is lowered by the binning process. Therefore, the enlargement process is performed so that the number of pixels is the same as the number of pixels of the first and second radiation image signals before the binning process. Specifically, for example, interpolation may be performed to increase the number of pixels.

  Then, the initial reconstructed image subjected to the enlargement process by the enlargement processing unit 30i is stored in the initial reconstructed image memory 30e. The subsequent processing is the same as that described in the above embodiment.

  In the above description, the binning process is performed on both the first and second radiographic image signals. However, the second radiographic image signal is obtained by binning reading instead of the binning process. You may do it. Binning readout refers to a method in which the charge signals of a plurality of pixels are added together and read out instead of reading out the charge signals of a large number of pixels constituting the radiation image detector 11a one by one. Also, for the first radiation image signal, since it is desired to maintain high-resolution information, it is not suitable to perform binning readout, and it is desirable to perform binning processing as described above.

  FIG. 8 shows an internal configuration of the computer 36 according to the embodiment for performing the binning reading described above. As shown in FIG. 8, the computer 36 according to the embodiment that performs binning readout has a readout control unit 30 j that controls to perform binning readout when acquiring the second radiation image signal, and the first radiation image signal. A binning processing unit 30k for performing binning processing and an enlargement processing unit 30i.

  In the case of this embodiment, when the second radiation image signal is read from the radiation image detector 11a, a control signal is output from the read control unit 30j to the radiation image detector 11a, and the radiation signal is output according to the control signal. The second radiation image signal is read out from the image detector 11a by binning readout and stored in the low-dose image memory 30a. As for the number of pixels to be added at the time of binning reading, the number of pixels corresponding to a desired cutoff frequency is set in advance in the reading control unit 30i.

  Then, the first radiation image signal read out from the high-dose image memory 30b and subjected to the binning process in the binning processing unit 30k and the second radiation image signal read out by the binning readout are the FBP method reconstruction unit. 30d, and the FBP method reconstructing unit 30d uses the first radiation image signal and the second radiation image signal to perform reconstruction processing by the FBP method to generate an initial reconstructed image. .

  Next, the initial reconstructed image generated by the FBP method reconstruction unit 30d is output to the enlargement processing unit 30i, and the enlargement processing unit 30i performs enlargement processing.

  Then, the initial reconstructed image subjected to the enlargement process by the enlargement processing unit 30i is stored in the initial reconstructed image memory 30e. The subsequent processing is the same as that described in the above embodiment.

  In the above-described embodiment, both the radiation image detector and the radiation source are configured to rotate. For example, when a large number of radiation image detectors are provided around the subject P, the radiation source is provided. Only need to rotate. Conversely, when many radiation sources are provided around the subject P, only the radiation image detector needs to be rotated. Even in such a configuration, the present invention can be applied.

  Moreover, although the said embodiment applies the radiographic imaging apparatus of this invention to the radiographic CT image imaging system which image | photographs CT image of a to-be-photographed head and chest, a to-be-photographed object is not restricted to these, For example, The present invention may be applied to a radiation CT image capturing system that captures a CT image of a breast of a subject.

DESCRIPTION OF SYMBOLS 1 Imaging device 10 Radiation source 11 Radiation detection part 11a Radiation image detector 11b Signal processing part 12 C arm 15 Rotation drive part 30, 35 Computer 30a Low-dose image memory 30b High-dose image memory 30c LPF processing part 30d LBP method reconstruction part 30e Initial reconstructed image memory 30f Successive approximation method reconstructing unit 30g Imaging control unit 30h, 30k Binning processing unit 30i Enlargement processing unit 30j Read control unit 31 Monitor

Claims (13)

A radiation image detector that detects radiation emitted from a radiation source and transmitted through the subject and outputs a radiation image signal representing a radiation image of the subject, and at least one of the radiation source circulates around the subject The radiographic image signal for each radiographing angle output from the radiographic image detector by irradiating the subject with the radiation for each radiographing angle is acquired, and regenerated based on the radiographic image signal for each radiographing angle. In a radiographic imaging method for performing a configuration and generating a tomographic image of the subject,
The radiation source is controlled so that a first dose of radiation is emitted at some of the imaging angles, and a second dose of radiation that is lower than the first dose at other imaging angles. Control the radiation source to be emitted,
The first radiation image signal output from the radiation image detector by irradiation with the first dose of radiation and the second radiation image output from the radiation image detector by irradiation with the second dose of radiation. And based on the signal, reconstruct using an analytical method that is a solution based on analytics to generate an initial reconstructed image, and based on the initial reconstructed image and the first radiation image signal, A radiographic imaging method comprising: generating a tomographic image of the subject by performing reconstruction using a statistical method which is a solution based on statistics different from the analytical method. A radiation source that emits radiation; a radiation image detector that detects radiation emitted from the radiation source and transmitted through a subject; and outputs a radiation image signal representing a radiation image of the subject; and the radiation source and the radiation image A rotation drive unit that circulates at least one of the detectors around the subject, and the radiographic image detector output by the radiation image detector by irradiating the subject with the radiation at predetermined radiographic angles. In a radiographic imaging apparatus comprising a reconstruction unit that acquires a radiographic image signal and performs reconstruction based on the radiographic image signal for each imaging angle to generate a tomographic image of the subject.
The radiation source is controlled so that a first dose of radiation is emitted at some of the imaging angles, and a second dose of radiation that is lower than the first dose at other imaging angles. A radiation source control unit for controlling the radiation source to be emitted;
The reconstruction unit outputs the first radiation image signal output from the radiation image detector by the irradiation of the first dose of radiation and the radiation image detector by the irradiation of the second dose of radiation. Based on the second radiographic image signal, reconstruction is performed using an analytical method which is a solution based on analytics to generate an initial reconstructed image, and the initial reconstructed image and the first radiographic image are generated. A radiological image characterized by generating a tomographic image of the subject by performing reconstruction based on a signal and using a statistical method that is a solution based on statistics different from the analytical method Shooting device. The radiographic imaging apparatus according to claim 2, wherein the reconstruction unit uses an FBP (Filtered Back Projection) method, an FFT (Fast Fourier Transform) method, or a convolution method as the analytical method. The radiographic image capturing apparatus according to claim 2, wherein the reconstruction unit uses a successive approximation method or an iterative method as the statistical method. The high frequency removal part which performs the process which removes a high frequency component with respect to the said initial reconstructed image or said 1st radiographic image signal, and said 2nd radiographic image signal is provided. The radiographic imaging apparatus of Claim 1. 5. The radiographic image capturing apparatus according to claim 2, further comprising a reduction processing unit that performs reduction processing on the first radiographic image signal and the second radiographic image signal. 6. An enlargement processing unit that performs an enlargement process on an initial reconstructed image generated based on the first radiographic image signal and the second radiographic image signal subjected to the reduction process, The radiographic imaging device according to claim 6. The radiation image detector is composed of a large number of pixels,
A readout control unit for performing binning readout by adding and reading out the charge signals of the plurality of pixels when reading out the second radiation image signal from the radiation image detector;
5. The radiographic image capturing apparatus according to claim 2, further comprising: a reduction processing unit that performs a reduction process on the first radiographic image signal. Enlarging an initial reconstructed image generated based on the first radiographic image signal subjected to the reduction process and the second radiographic image signal output from the radiographic image detector by the binning readout. The radiographic imaging apparatus according to claim 8, further comprising an enlargement processing unit that performs processing. The radiographic image capturing apparatus according to claim 6, wherein the reduction process is a binning process. The radiation source control unit controls the radiation source so that the number of imaging angles at which the first dose of radiation is emitted is smaller than the number of imaging angles at which the second dose of radiation is emitted. The radiographic imaging apparatus according to claim 2, wherein the radiographic imaging apparatus is controlled. 12. The number of imaging angles at which the first dose of radiation is emitted is 1/5 or more and 1/3 or less of the imaging angle at which the second dose of radiation is emitted. Radiation imaging device. The radiographic imaging apparatus according to claim 2, wherein the second dose is 1/8 or more and 1/4 or less of the first dose.

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