JP5210048B2 - Energy subtraction processing apparatus and method, and program - Google Patents

Description

  The present invention relates to an energy subtraction processing apparatus, method, and program which are performed using a low energy image and a high energy image acquired by a double exposure method.

  Conventionally, subtraction processing is performed using two radiation images having different energies to generate a bone part image representing a bone shadow and a soft part image representing a soft tissue. As a method of acquiring two radiation images having different energies, a single exposure method in which a copper plate is sandwiched between two radiation detectors and two radiation images having different energies are acquired by one exposure (for example, a patent) Document 1-3) and a two-time exposure method for acquiring a high-energy image photographed with high-energy X-rays and a low-energy image photographed with low-energy X-rays have been proposed (for example, see Patent Document 4). ).

  In particular, Patent Documents 1-4 disclose various setting / adjustment techniques for parameters (load subtraction count) in energy subtraction processing. Specifically, in Patent Document 1, in a single exposure method, the subject's contrast fluctuation is estimated by the ratio of the relative contrast of the image (reciprocal ≡ Gp value of the image histogram width) for each subject or pixel. A method of applying appropriate parameters for each is disclosed. Patent Document 2 discloses a method of changing parameters according to the thickness of a subject in a single exposure method. Patent Document 3 discloses changing parameters based on the width of a histogram (signal distribution) of a high energy image.

On the other hand, in Patent Document 4, an energy subtraction coefficient corresponding to a tube voltage combination table prepared in advance is provisionally determined by a combination of photographing tube voltages of a high-energy image and a low-energy image of the two-exposure method, A method is disclosed in which energy subtraction is performed using a temporary coefficient and parameters are optimized so that the edge strength of the soft part image is minimized.
JP-A-6-22219 Japanese Patent Laid-Open No. 10-118056 Japanese Patent Laid-Open No. 2002-359781 JP 2003-37778 A

  Incidentally, the beam hardening described in Patent Documents 1-3 of the single exposure method described above is a phenomenon that occurs even in the double exposure method. Therefore, when performing the energy subtraction process by the double exposure method shown in Patent Document 4, the parameter setting / adjustment method disclosed in Patent Document 1-3 is adopted, and the energy subtraction process by an appropriate parameter is performed. Can be considered. However, the effects of beam hardening are completely different between the single exposure method and the double exposure method, and the optimization of the parameters of the single exposure method shown in Patent Document 1-3 is performed as it is. However, there is a problem that the component separation between the bone part and the soft part fails.

  Accordingly, an object of the present invention is to provide an energy subtraction processing apparatus, method, and program capable of reducing image quality degradation of a subtraction image due to beam hardening, radiation scattering, or the like in the double exposure method. .

  The energy subtraction device of the present invention includes an image acquisition unit that acquires a high energy image and a low energy image when a subject is irradiated with radiation of different energy from a radiation source, a pixel density value of the high energy image, and a high energy The first load subtraction coefficient associated with the first load subtraction coefficient multiplied by the image and the first load subtraction coefficient associated with the first load subtraction coefficient nonlinearly as the density value increases and / or the pixel density value of the high energy image A table database storing a second load subtraction table associated with the second load subtraction coefficient multiplied by the low energy image so that the second load subtraction coefficient increases nonlinearly as the density value increases; and image acquisition means Depending on the density value of each pixel of the high energy image and / or low energy image acquired by Parameter setting means for setting the first load subtraction coefficient and / or the second load subtraction coefficient for each pixel using the first load subtraction table and / or the second load subtraction table, and the first set by the parameter setting means Image processing means for performing energy subtraction processing using a high energy image and a low energy image based on the load subtraction coefficient and / or the second load subtraction coefficient is provided.

  The energy subtraction processing method of the present invention acquires a high energy image and a low energy image when a subject is irradiated with radiation of different energy from a radiation source, and multiplies the pixel density value of the high energy image by the high energy image. The first load subtraction table associated with the first load subtraction coefficient so that the first load subtraction coefficient becomes nonlinearly smaller as the density value becomes higher, and / or the pixel density value of the high energy image and the low energy image. Using the second load subtraction table associated with the second load subtraction coefficient to be multiplied by the second load subtraction coefficient so that the second load subtraction coefficient increases nonlinearly as the density value increases, the acquired high energy image and / or low The first load subtraction coefficient and / or the second load subtraction coefficient for each pixel according to the density value of each pixel of the energy image Set, on the basis of the first load subtraction factor and / or the second load subtraction coefficient set, and is characterized in carrying out the energy subtraction processing using a high energy image and a low energy image.

  The energy subtraction processing program of the present invention acquires a high energy image and a low energy image when a subject is irradiated with radiation of different energies from a radiation source, and obtains a pixel density value and a high energy image of the high energy image. The first load subtraction coefficient associated with the first load subtraction coefficient multiplied by the image and the first load subtraction coefficient associated with the first load subtraction coefficient nonlinearly as the density value increases and / or the pixel density value of the high energy image Using the second load subtraction table associated with the second load subtraction coefficient multiplied by the low energy image so that the second load subtraction coefficient increases nonlinearly as the density value increases, the acquired high energy image and / Or a first load subtraction coefficient for each pixel and / or according to the density value of each pixel of the low energy image Alternatively, a second load subtraction coefficient is set, and energy subtraction processing using a high energy image and a low energy image is performed based on the set first load subtraction coefficient and / or second load subtraction coefficient. To do.

  Here, a high or low density value means that the density value in each image is high or low, but the density value increases when the local thickness of the subject is small, and the local thickness of the subject is large. Sometimes the density value is low. For this reason, a high or low density value means that the local thickness of the subject is small or large.

  The parameter setting means may set the first load subtraction coefficient only for the first load subtraction table, or may set the second load subtraction coefficient using only the second load subtraction table. Alternatively, the first load subtraction coefficient and the second load subtraction coefficient may be set using both the first load subtraction table and the second load subtraction table. When only one of the load subtraction tables is used, a constant predetermined value is set for the other load subtraction coefficient regardless of the concentration value.

  Furthermore, the energy subtraction processing device may further include a thickness detecting unit that detects the thickness of the subject. At this time, the parameter setting means uses the first load subtraction table and / or the second load subtraction table and / or the second load set based on the thickness of the subject detected by the thickness detection means. The subtraction coefficient may be adjusted.

  In particular, it is preferable that the parameter setting means adjust the value of the first load subtraction coefficient larger as the thickness of the subject increases.

  Further, the thickness detecting means may be any method as long as it can detect the thickness of the subject. For example, the thickness detecting means calculates the width of the signal distribution of the high energy image and the low energy image as the thickness information. Also good. At this time, the parameter setting means may have a function of adjusting the parameter using the ratio of the signal distribution widths of the high energy image and the low energy image calculated by the thickness detection means. The signal distribution width is determined by, for example, a scale factor value (reciprocal of the width of the histogram, Gp value) or a latitude value (L value = 4 / Gp), the width of the density histogram, and the maximum and minimum of each energy image. It means the difference in density.

  Moreover, the energy subtraction processing apparatus may further include a tube voltage detection unit that detects a tube voltage applied to the radiation source. At this time, the table database stores a plurality of the first load subtraction tables and / or a plurality of the second load subtraction tables for each tube voltage at the time of acquisition of the low energy image. Based on the tube voltage detected by the voltage detection means, the first load subtraction table and / or the second load from among the plurality of first load subtraction tables and / or the plurality of second load subtraction tables. A subtraction table may be selected and the first load subtraction coefficient and / or the second load subtraction coefficient may be set.

  In particular, it is preferable that the plurality of first load subtraction tables are set such that the first load subtraction table has a smaller offset value as the first load subtraction table has a higher tube voltage at the time of acquiring a low energy image.

  According to the energy subtraction processing apparatus, method, and program of the present invention, a high energy image and a low energy image are obtained when a subject is irradiated with radiation of different energy from a radiation source, and a pixel density value of the high energy image is acquired. And the first load subtraction coefficient that is multiplied by the high energy image and the first load subtraction coefficient associated with the first load subtraction coefficient nonlinearly as the density value increases and / or the pixels of the high energy image Using the second load subtraction table associated with the density value and the second load subtraction coefficient multiplied by the low energy image so that the second load subtraction coefficient increases nonlinearly as the density value increases, the acquired high value is obtained. First load subtraction for each pixel according to the density value of each pixel of the energy image and / or low energy image By setting the number and / or the second load subtraction coefficient, and performing the energy subtraction process using the high energy image and the low energy image based on the set first load subtraction coefficient and / or the second load subtraction coefficient Since it is possible to set parameters in consideration of the effects of beam hardening and radiation scattering by the subject, it is possible to obtain an appropriate energy subtraction result.

  The apparatus further includes a thickness detection unit for detecting the thickness of the subject, and the parameter setting unit sets the first load subtraction table and / or the second load subtraction table based on the thickness of the subject detected by the thickness detection unit. The first load subtraction coefficient and / or the second load subtraction coefficient is adjusted. In particular, if the parameter setting means adjusts the value of the first load subtraction coefficient larger as the subject thickness increases, the subject Even when the influence of beam hardening differs depending on the subject depending on the thickness (physique) of the subject, it is possible to obtain a stable energy subtraction result by adjusting to the optimum load subtraction coefficient.

  Furthermore, it further has a tube voltage detecting means for detecting a tube voltage applied to the radiation source, and the table database includes a plurality of the first load subtraction tables and / or a plurality of the plurality of the first load subtraction tables for each tube voltage at the time of acquiring the low energy image. The second load subtraction table is stored, and the parameter setting means has a plurality of first load subtraction tables and / or a plurality of second load subtraction tables based on the tube voltage detected by the tube voltage detection means. Any one of the first load subtraction table and / or the second load subtraction table is selected, and the first load subtraction coefficient and / or the second load subtraction coefficient is set. This is the case where the influence of beam hardening varies from one radiograph to another due to changes in tube voltage (radiation energy) when acquiring low-energy images Even when the signal distribution width ratio cannot be adjusted properly, it is possible to obtain a stable energy subtraction result by adjusting to the optimum load subtraction coefficient by preparing a plurality of load subtraction tables. .

  Hereinafter, embodiments of an energy subtraction processing apparatus of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing a preferred embodiment of the energy subtraction processing apparatus of the present invention. The configuration of the energy subtraction processing device 1 as shown in FIG. 1 is realized by executing the energy subtraction processing program read into the auxiliary storage device on a computer (for example, a personal computer). At this time, the energy subtraction processing program is stored in an information storage medium such as a CD-ROM, or distributed via a network such as the Internet and installed in a computer.

  The energy subtraction processing apparatus 1 includes an image acquisition unit 10, an alignment unit 20, an image processing unit 30, an image adjustment unit 40, and the like. The image acquisition means 10 acquires a radiographic image detected by a radiation detector, and in particular, a high energy image (diagnostic image) HP and a low energy image (photographed using radiation having different energies). Reference image) SP is acquired. For example, in chest front imaging, the high energy image HP is an image taken when a tube voltage of about 100 to 140 kVp is applied to the radiation source, and the low energy image LP is applied to the radiation source of about 50 to 80 kVp. It is an image that was sometimes taken. The gradation processing means 15 performs gradation correction of density values as preprocessing for the high energy image HP and the low energy image LP.

  The alignment unit 20 performs alignment between the high energy image HP and the low energy image LP using a known alignment method such as nonlinear distortion and deformation of the low energy image LP. Thereby, the separation performance of soft tissue and bone can be improved by the energy subtraction process.

  The image processing unit 30 performs an energy subtraction process using the high energy image HP and the low energy image LP that have been subjected to the alignment process by the alignment unit 20. Specifically, the image processing unit 30 includes a subtraction processing unit 31 that performs subtraction processing and a parameter setting unit 32 that sets parameters in the subtraction processing.

The subtraction processing means 31 has a function of generating a soft part image SP indicating the soft part of the subject from which the bone has been removed and a bone part image BP indicating the bone part of the subject using the high energy image HP and the low energy image LP. doing. In general, the subtraction image Psub is represented by the difference between the high energy image HP obtained by integrating the first load subtraction coefficient Ka and the low energy image LP obtained by integrating the second load subtraction coefficient Kb.
Psub = Ka · HP−Kb · LP + Kc (1)
In Equation (1), Ka is a first load subtraction coefficient, Kb is a second load subtraction coefficient, and Kc is a predetermined offset value.

  And the subtraction process means 31 produces | generates soft part image SP as a subtraction image Psub by calculating Formula (1). Thereafter, the subtraction processing means 31 obtains the bone part image BP by subtracting the soft part image SP from the high energy image HP (BP = HP-SP).

  The parameter setting unit 32 sets various parameters Ka, Kb, and Kc in the above equation (1). Here, the parameter setting means 32 sets the first load subtraction coefficient Ka according to the density value based on the first load subtraction table KT1 for each pixel of the high energy image HP. Alternatively, the parameter setting unit 32 sets the second load subtraction coefficient Kb according to the density value based on the second load subtraction table KT2 for each pixel of the high energy image HP.

  Here, FIG. 2A is a graph showing an example of the first load subtraction table KT1 in which the relationship between the pixel density value and the first load subtraction coefficient Ka is stored. In FIG. 2A, the first load subtraction coefficient Ka has a non-linear characteristic that increases as the density value decreases and decreases as the density value increases. On the other hand, FIG. 2B is a graph showing an example of the second load subtraction table KT2 in which the relationship between the pixel density value and the second load subtraction coefficient Kb is stored. In FIG. 2B, the second load subtraction coefficient Kb has a non-linear characteristic that decreases as the density value decreases and increases as the density value increases. More specifically, in FIGS. 2A and 2B, the non-linear characteristic has a small gradient in the high and low concentration regions and a large gradient in the intermediate region between the high and low concentration values. ing.

2A and 2B are generated (adjusted) as follows, for example. First, considering that the normalization condition (GpH) of the high energy image HP is matched with the normalization condition (GpL) of the low energy image LP, the soft part image SP that is the subtraction image Psub is expressed by the following equation (2). Can be represented.

In Expression (2), GpL / GpH × (HP−C) + C is a high-energy image after correction of the normalization conditions, K ₁ is a contrast coefficient for the soft part image SP, and TA is a bone disappearance degree in the soft part image SP. Energy sub-coefficient, C is a median density value (for example, 512 for 10 bits), and TC is an offset value for stabilizing the density of the soft part image SP. It can be seen from Equations (1) and (2) that Ka = K ₁ × TA × (GpL / GpH) and Kb = K ₁ . Similarly, the bone part image BP can be expressed as the following formula (3).

In Expression (3), K ₂ is a contrast coefficient for the bone image BP, and TB is an energy sub coefficient for controlling the disappearance of the soft part in the bone image BP.

Here, when the bone part image BP is generated, the soft part image SP is subtracted from the addition image AP obtained by mixing the high energy image HP and the low energy image LP at a mixing ratio of M1: M2 (BP = AP− Think about SP). First, the addition image AP can be expressed as the following formula (4).

Note that when M2 = 0, the bone part image BP is calculated by subtracting the soft part image SP from the high energy image HP. Therefore, the bone part image BP = AP-SP is
It becomes. From equations (3) and (5), K ₁ is
It becomes.

Substituting the above equation (6) into Ka = K ₁ × TA × (GpL / GpH), Kb = K ₁ ,
It becomes. Then, the load subtraction tables KT1 and KT2 having nonlinear characteristics as shown in FIG. 2A and FIG. 2B are generated using the equations (7) and (8) by adjusting the energy sub-coefficients TA and TB, and the table database TB Is remembered.

  Specifically, by adjusting the values of the energy sub-coefficients TA and TB, (1) a table KT1 in which the first load subtraction coefficient Ka has nonlinear characteristics (FIG. 2A) and the second load subtraction coefficient Kb is a predetermined value. (2) Table KT2 in which the second load subtraction coefficient Kb has nonlinear characteristics (FIG. 2B) and the first load subtraction coefficient Ka is a predetermined value, (3) the first load subtraction coefficient Ka and the second load subtraction coefficient Kb Either of the tables KT1 and KT2 having nonlinear characteristics (FIGS. 2A and 2B) is generated and stored in the table database TB. Note that, in any of the above forms (1) to (3), the energy sub-coefficients TA and TB are non-linear graphs as shown in FIG. 2A with respect to the density value.

  That is, the parameter setting unit 32 may set only the first load subtraction coefficient Ka according to the density value using the first load subtraction table KT1. At this time, the second load subtraction coefficient Kb is set to a predetermined value. Alternatively, the parameter setting unit 32 may set only the first second load subtraction coefficient Kb according to the density value using the first load subtraction table KT1. At this time, the first load subtraction coefficient Ka is set to a predetermined value. Further, the parameter setting means 32 sets both the first load subtraction coefficient Ka and the second load subtraction coefficient Kb according to the concentration value by using both the first load subtraction table KT1 and the second load subtraction table KT2. You may make it do.

  By setting the load subtraction coefficients Ka and Kb using the load subtraction tables KT1 and KT2 having nonlinear characteristics as shown in FIGS. 2A and 2B, the contrast of the subtraction image Psub is reduced due to the effect of beam hardening. It is possible to reduce image quality degradation. Specifically, in the case of the double exposure method, the contrast ratio between the high energy image HP and the low energy image LP increases as the thickness of the subject increases due to the effect of beam hardening. That is, when radiation passes through a substance, the number of photons (dose) of the radiation decreases. The lower the energy, the larger the attenuation coefficient, and the higher the energy, the smaller the attenuation coefficient and the greater the transparency. Since the low energy component of radiation is attenuated more than the high energy component, the lower the energy component of the radiation is attenuated, the higher the ratio of the high energy component (the higher the average energy). This phenomenon is called beam hardening. FIG. 3 shows that the attenuation coefficient decreases as the radiation energy (tube voltage) increases also in the bone part and the soft part.

  In addition, the higher the atomic number of a substance and the higher the density, the greater the attenuation coefficient of radiation. In other words, the degree of beam hardening differs depending on the ratio of the substance present on the radiation passage path, and even in the same subject, the region having a larger local thickness is more strongly affected by beam hardening. Specifically, as shown in FIG. 4, the rate of attenuation is large when the effect of beam hardening is large, and the rate of attenuation is small when the effect is small, with respect to the energy spectrum of radiation irradiated to the subject.

  The photoelectric effect and the Compton effect (Compton scattering or incoherent scattering) are raised as factors causing the beam hardening described above. The photoelectric effect is a phenomenon in which inner-shell electrons having strong bonds with atomic nuclei are ejected as free electrons (photoelectrons). At this time, the incident radiation photon loses all its energy and disappears, and the outer orbital electrons transition to the vacant inner electron trajectory. For this reason, characteristic X-rays having energy between the energy levels before and after that are emitted as secondary X-rays (known as fluorescent radiation). As shown in FIG. 5, the photoelectric effect appears remarkably with radiation having a relatively low photon energy (about 50 keV or less), and becomes larger as the density and atomic number of the substance increases.

  The Compton effect is a phenomenon that appears as a particle of radiation. When an incident radiation photon collides with a free electron or an outer shell electron with a weak bond with a nucleus, it gives a part of energy to the electron and counteracts it. This is a phenomenon that jumps out of the orbit as a jumping electron, and changes its direction by becoming a long wavelength radiation with a smaller energy. The Compton effect is a phenomenon controlled by photon energy and material density (the higher the density, the stronger). As shown in FIG. 5, in the relatively high energy radiation region (about 50 keV or more), the atomic number Except for high substances (such as heavy metals and iodine), it appears stronger than the photoelectric effect. Compton scattering reduces the image contrast.

  FIG. 6 shows a region where the local thickness of the subject is small (FIG. 6A) and the local area of the subject when the low-energy image LP is taken and acquired at 60 kVp and the high-energy image HP is obtained at 120 kVp in the double exposure method. It is a graph about the area | region (FIG. 6B) with a large thickness. When the local thickness of the subject in FIG. 6A is small, the energy spectra of the low energy image LP and the high energy image HP are close to each other. For example, while the average energy of the low energy image LP is 33.9 keV, the average energy of the high energy image HP is 45.9 keV, and the difference in average energy is small. Accordingly, the contrast ratio between the high energy image HP and the low energy image LP becomes small.

  On the other hand, when the local thickness of the subject in FIG. 6B is large, beam hardening occurs strongly in the subject, and the energy spectra of the high energy image HP and the low energy image LP are separated. At this time, the average energy of the low energy image LP is, for example, 42.9 keV, whereas the average energy of the high energy image HP is 63.4 keV, indicating that the difference in average energy is large. Accordingly, the contrast ratio between the high energy image HP and the low energy image LP is increased.

  As shown in FIGS. 6A and 6B, the effect of beam hardening does not occur so much in a portion where the local thickness of the subject is small, but it is strongly caused in a portion where the local thickness of the subject is large. . That is, as the local thickness of the subject increases, the energy spectrum of the high energy image HP and the low energy image LP are separated from each other, and the contrast ratio between the high energy image HP and the low energy image LP increases.

  By the way, when the local thickness of the subject is large, the density value in each energy image LP, HP is low, and when the local thickness of the subject is small, the density value is high. Therefore, the degree of influence of beam hardening due to the above-described thickness can be determined by the level of the pixel density value. Therefore, as shown in FIG. 2A, the parameter setting means 32 increases the first load subtraction coefficient Ka as the density value decreases, and sets the first load subtraction coefficient Ka as the density value increases. Alternatively, as shown in FIG. 2B, the parameter setting means 32 decreases the second load subtraction coefficient Kb as the density value decreases, and sets the second load subtraction coefficient Kb as the density value increases. Thereby, it is possible to prevent a decrease in contrast due to the effect of beam hardening, and to reduce image quality deterioration of the subtraction image Psub (soft part image SP and bone part image BP).

  Note that this relationship is opposite to the one-time exposure method, and component separation cannot be performed well even using a nonlinear table applied in the conventional one-time exposure method. Specifically, FIG. 7 is a joint histogram in which the horizontal axis represents the density value of the high energy image HP and the vertical axis represents the density value of the low energy image LP, and the frequency is graphed. FIG. 7B shows a result obtained by a one-shot method (one-shot method). The density is normalized to 0 to 127, and becomes a straight line when there is no influence of beam hardening corresponding to the contrast ratio between the high energy image HP and the low energy image LP.

  In the case of the double exposure method of FIG. 7A, the contrast ratio between the low energy image LP and the high energy image HP is non-linearly shifted to the lower side of the straight line in the region where the density value is small due to the effect of beam hardening. On the other hand, in the case of the single exposure method of FIG. 7B, the contrast ratio between the low energy image LP and the high energy image HP is non-linearly shifted to the upper side of the straight line in the region where the density value is small due to the effect of beam hardening. . Thus, it can be seen that the characteristics are reversed between the single exposure method and the double exposure method. Therefore, it can be seen that component separation cannot be performed well even using a non-linear table applied in the conventional single exposure method.

  FIG. 8 is a flowchart showing a preferred embodiment of the energy subtraction processing method of the present invention. The energy subtraction processing method will be described with reference to FIGS. First, the high-energy image HP and the low-energy image LP are acquired by the image acquisition means 10 (step ST1). Thereafter, the gradation processing means 15 adjusts the gradation of the high energy image HP and the low energy image LP. Further, the low energy image is nonlinearly distorted and deformed by the alignment means 20, whereby the high energy image HP and the low energy image LP are aligned (step ST2).

  Thereafter, the soft part image SP is generated based on the above formula (1) in the image processing means 30 (step ST3). Specifically, in the parameter setting means 32, the first load subtraction coefficient Ka or the second load subtraction coefficient Kb corresponding to the density for each pixel of the high energy image HP is set to the first load subtraction table KT1 or the second load subtraction table KT2. (See step ST3-1, FIG. 2A, FIG. 2B). Then, the soft part image SP is generated using the first load subtraction coefficient Ka or the second load subtraction coefficient Kb determined by the subtraction processing means 31 (step ST3-2). The operations of steps ST3-1 to ST3-2 are repeated for all the pixels of the high energy image HP (step ST3-3), and the soft part image SP is generated as the subtraction image Psub.

  Next, a bone part image BP is generated by subtracting the soft part image SP from the high energy image HP by the image processing means 30 (step ST4). Thereafter, a noise removal process by smoothing is performed by the image adjusting unit 40 (step ST5), and the soft part image SP and the bone part image BP are displayed on a display device or the like, or stored in a storage unit such as a hard disk device.

  Thus, by setting the load subtraction coefficients Ka and Kb using the load subtraction tables KT1 and KT2 having the nonlinear characteristics shown in FIG. 2A or FIG. 2B, a reduction in contrast due to the effect of beam hardening is prevented. Image quality deterioration of the subtraction image Psub (soft part image SP and bone part image BP) can be reduced.

  FIG. 9 is a block diagram showing a second embodiment of the energy subtraction processing apparatus of the present invention. The energy subtraction processing apparatus 100 will be described with reference to FIG. In addition, in the energy subtraction processing apparatus 100 of FIG. 9, the same code | symbol is attached | subjected to the site | part which has the same structure as the energy subtraction processing apparatus 1 of FIG. 1, and the description is abbreviate | omitted. The energy subtraction processing device 100 of FIG. 9 differs from the energy subtraction processing device 1 of FIG. 1 in that the load subtraction coefficients Ka and Kb are adjusted based on the thickness (physique) of the subject.

  The energy subtraction processing device 100 in FIG. 9 includes a thickness detection unit 140 that detects the thickness of the subject. The thickness detecting unit 140 detects the thickness of the subject, and in particular, the scale factor (reciprocal of the histogram width) GpH value of the high energy image HP and the scale factor (reciprocal of the histogram width) GpL value of the low energy image LP. It has a function of detecting the ratio GpH / GpL as thickness information.

  Below, it demonstrates that ratio GpH / GpL shows thickness information. FIG. 10A is a graph showing a histogram of a low energy image LP and a high energy image HP acquired when a subject with a small thickness (small physique) is photographed, and FIG. 10B shows a subject with a large thickness (large physique). It is a graph which shows the histogram of the low energy image LP and the high energy image HP which were sometimes acquired. As shown in FIGS. 10A and 10B, focusing on the change in the width of the histogram, the high energy image HP has a small variation with respect to the size of the physique, whereas the low energy image LP has a large variation with respect to the size of the physique. This is because when the subject is thick, the amount of X-ray radiation transmitted through the subject exposed to the radiation detector is reduced as compared to when the subject is thin.

  That is, the scale factor value GpH of the high-energy image HP hardly depends on the thickness of the subject, but the scale factor value GpL of the low-energy image LP varies greatly depending on the thickness of the subject. That is, as the thickness of the subject increases, the width of the histogram increases, the GpL value that is the reciprocal thereof decreases, and the Gp value ratio GpH / GpL increases. Therefore, the Gp value ratio GpH / GpL can be used as thickness information indicating the thickness of the subject.

  In addition, it is not limited to the case of estimating the physique based on the ratio GpH / GpL of the scale factor Gp value. For example, the height, weight or BMI (Body Mass Index) of the patient may be used, By measuring the distance between the source and the patient, the thickness of the subject may be directly measured, or the physique of the patient may be manually input by the photographer (doctor or radiographer). When the thickness detection means 140 detects thickness information by a method other than the ratio GpH / GpL of the scale factor Gp value, for example, when the thickness is detected such as a large physique, a standard person, or a small person, It may have a plurality of adjustment values (GpH / GpL values) associated with each other, and the adjustment value may be selected as the thickness information based on the inputted physique information.

  The parameter setting means 132 adjusts the first load subtraction coefficient Ka or the second load subtraction coefficient Kb set according to the density value using the first load subtraction table KT1 according to the thickness information of the subject. Specifically, the first load subtraction table KT1 is adjusted so that the offset value increases when the thickness is large, and the offset value decreases when the thickness is small. Note that the relationship between the GpH / GpL value and the offset value is preset, and has a proportional relationship, for example. Then, the subtraction processing means 131 uses the adjusted first load subtraction table KT1 and uses the first load subtraction coefficient TA (Ka) after adjustment according to the thickness, as shown in the following formulas (1) and (2). An image Ssub (= soft part image SP) is generated.

  In this way, by adjusting the first load subtraction coefficient Ka according to the thickness (physique) of the subject, it is possible to reduce the reduction in contrast of the subtraction image due to the influence of beam hardening caused by the physique difference. it can.

  In addition, although illustrated about the case where the 1st load subtraction coefficient Ka is adjusted as shown in Formula (2), you may make it adjust the 2nd load subtraction coefficient Kb. In this case, the second load subtraction coefficient Kb is adjusted so as to decrease as the thickness of the subject decreases.

  FIG. 12 is a block diagram showing a third embodiment of the energy subtraction processing apparatus of the present invention. The energy subtraction processing apparatus will be described with reference to FIG. In the energy subtraction processing device 200 of FIG. 12, parts having the same configuration as those of the energy subtraction processing device 100 of FIG. 9 are denoted by the same reference numerals and description thereof is omitted. The energy subtraction processing device 200 of FIG. 12 is different from the energy subtraction processing device 100 of FIG. 9 in that different load subtraction tables are used depending on the tube voltage value applied to the radiation source.

  The energy subtraction processing device 200 of FIG. 12 includes tube voltage detection means 240 that detects the tube voltage. The tube voltage detection means 240 acquires, for example, a tube voltage value applied to the radiation source from a radiation control unit (not shown) that controls the radiation source.

  On the other hand, the table database TB stores a plurality of first load subtraction tables KT11 to KT13 for each different tube voltage, and the parameter setting unit 232 acquires the low energy image LP detected by the tube voltage detection unit 240. One of the first load subtraction tables KT11 to KT13 is selected based on the tube voltage value at. Then, the parameter setting unit 232 sets the first load subtraction coefficient Ka using the selected first load subtraction tables KT11 to KT13. Specifically, as shown in FIGS. 13A to 13C, for example, three load subtraction tables KT11 to KT13 are stored corresponding to 60, 70, and 80 kVp. In the three load subtraction tables KT11 to KT13, the slopes of the curves are substantially the same, but the offset value of the first load subtraction coefficient is smaller as the first load subtraction table has a higher tube voltage when acquiring a low energy image. Is set as follows.

  Then, the parameter setting unit 232 determines which one of the load subtraction tables KT11 to KT13 to use depending on the tube voltage of the low energy image LP. Thereafter, the parameter setting unit 232 sets the first load subtraction coefficient Ka for each pixel using any of the determined load subtraction tables KT11 to KT13. Further, the parameter setting unit 232 adjusts the first load subtraction coefficient Ka set using the load subtraction tables KT11 to KT13 according to the thickness of the subject (see FIGS. 9 to 11).

  As described above, by adjusting the first load subtraction coefficient Ka according to the tube voltage at the time of acquiring the low energy image LP, it is possible to reduce image quality degradation of the subtraction image due to the effect of beam hardening in the low energy image LP.

  The reason why a plurality of load subtraction tables are prepared will be described below. First, the high-energy image HP is imaged by changing the tube voltage according to the patient's physique and the imaging purpose. As the tube voltage increases, the ratio of highly transmissive energy components increases. For this reason, the logarithmic exposure amount of the main histogram increases, the histogram width becomes narrower, and the contrast of the high energy image HP becomes smaller. That is, the scale factor Gp value, which is the reciprocal of the histogram width, increases.

  FIG. 14 is a graph showing a histogram when the same subject is photographed by changing the tube voltage when acquiring the high energy image HP to 100 to 140 kVp. As shown in FIG. 14, the higher the tube voltage, the greater the transmission due to the increase in X-rays with high energy components. For this reason, in the histogram of the radiographic image, the histogram peak of the region directly irradiated with X-rays (the missing portion) approaches the peak of the main histogram of the portion that has passed through the subject. Therefore, the width of the histogram is reduced and the scale factor Gp value is increased.

  Here, when the tube voltage at the time of acquiring the low energy image LP is fixed to 60 kVp, the Gp value ratio GpH / GpL increases as the tube voltage of the high energy image HP increases. However, in this case, the deterioration of the separation performance due to the change in tube voltage can be reduced by adjusting the Gp value ratio GpH / GpL as performed by the parameter setting means 132 described above (FIGS. 9 to 11). reference). That is, the tube voltage change of the high energy image HP can be adjusted using the ratio GpH / GpL of the Gp values of the high energy image HP.

  By the way, in the low energy image LP, the influence of the scattered radiation is caused in the same manner as the influence of the scattered radiation in the high energy image HP. Specifically, the histogram width of the low energy image LP becomes narrower and the GpL value becomes larger as the tube voltage becomes higher. That is, since the contrast of the low energy image LP decreases as the tube voltage increases, it is necessary to adjust the first load subtraction coefficient Ka multiplied by the high energy image HP accordingly. At this time, it is conceivable to adjust the change in the tube voltage of the low energy image LP with the Gp value ratio GpH / GpL described above.

  However, when the change in the tube voltage of the low energy image LP is adjusted by the ratio GpH / GpL of the Gp value, the bone remains in the soft part image, and the soft part and the bone part are not sufficiently separated, and the low energy image LP It has been found that this tendency becomes more pronounced as the tube voltage increases.

  Here, FIG. 15 is a graph showing an energy spectrum when the same subject is photographed by changing the tube voltage when acquiring the low energy image LP to 60 to 80 kVp. As shown in FIG. 15, when the tube voltage of the low energy image LP is increased, the ratio of the energy component of 50 keV or more is 9% at 60 kVp, 22% at 70 kVp, and 32% at 80 kVp. This is because, among the photoelectric effect and the Compton effect, which are the factors of the beam hardening described above, the influence of Compton scattering becomes stronger than the influence of the photoelectric effect in accordance with the increase in the tube voltage of the low energy image LP. In other words, the tube voltage value used when acquiring the low-energy image LP is a mixture of both the photoelectric effect and the Compton effect, and the degree of influence differs depending on the tube voltage value (see FIG. 3). It was found that proper correction cannot be performed.

  Therefore, the adjustment of the coefficient due to the change in the tube voltage of the low energy image LP is not the adjustment by the Gp value ratio GpH / GpL, but a plurality of load subtraction tables KT11 to KT13 are separately stored, and an appropriate value corresponding to each tube voltage is stored. A first load subtraction coefficient Ka is set using the load subtraction table. Thereby, it is possible to reliably reduce the image quality deterioration of the subtraction image.

  Conventionally, a parameter (a uniform scalar amount in the image) is selected using a look-up table in accordance with the tube voltage of the high energy image and the low energy image. Is a diagnostic image, and it is thought that the factor that the contrast greatly fluctuates is due to the individual's physique fluctuation rather than the tube voltage, which is different from FIGS.

  Further, in FIG. 13, the case where the first load subtraction coefficient Ka is set using the first load subtraction tables KT11 to KT13 is illustrated, but the second load subtraction coefficient Kb may be adjusted. In this case, a plurality of second load subtraction tables are prepared such that the second load subtraction coefficient Kb becomes smaller as the tube voltage at the time of acquisition of the low energy image LP becomes smaller as the second load subtraction coefficient Kb.

  According to each of the above embodiments, a high energy image HP and a low energy image LP obtained by irradiating a subject with radiation of different energies from a radiation source are acquired, and the pixel density value of the high energy image and the high energy image are acquired. The first load subtraction table KT1 and / or the high energy image HP associated with the first load subtraction coefficient Ka multiplied by HP so that the first load subtraction coefficient Ka becomes non-linearly smaller as the density value becomes higher. Using the second load subtraction table KT2 associated with the pixel density value and the second load subtraction coefficient Kb multiplied by the low energy image LP so that the second load subtraction coefficient increases nonlinearly as the density value increases. The first load subtraction unit for each pixel according to the density value of each pixel of the acquired high energy image HP and / or low energy image LP By setting Ka and / or the second load subtraction coefficient Kb and performing the energy subtraction process using the set first load subtraction coefficient Ka and / or the second load subtraction coefficient Kb, the beam hardening and the radiation of the subject Since it is possible to set a parameter in consideration of the influence of scattering or the like, an appropriate energy subtraction result can be obtained.

  Further, as shown in FIG. 9, the apparatus further includes a thickness detection unit 140 that detects the thickness of the subject, and the parameter setting unit 132 uses the first load subtraction table KT1 and the first load subtraction table KT1 based on the thickness of the subject detected by the thickness detection unit 140. The first load subtraction coefficient Ka and / or the second load subtraction coefficient Kb set using the second load subtraction table KT2 is adjusted. In particular, the parameter setting means 132 increases as the subject thickness increases. If the value of the load subtraction coefficient is greatly adjusted, even if the influence of beam hardening differs depending on the subject depending on the thickness (physique) of the subject, the energy is adjusted to the optimum load subtraction coefficient and stable energy Subtraction results can be obtained.

  Furthermore, as shown in FIG. 12, it further has a tube voltage detecting means 240 for detecting a tube voltage applied to the radiation source, and the table database TB includes a plurality of first voltages for each tube voltage at the time of acquiring the low energy image LP. 1 load subtraction tables KT11 to KT13 and / or a plurality of second load subtraction tables are stored, and the parameter setting means 232 uses a plurality of first loads based on the tube voltage detected by the tube voltage detection means 240. Any one of the first load subtraction tables KT11 to KT13 and / or the second load subtraction table is selected from the subtraction tables KT11 to KT13 and / or the plurality of second load subtraction tables, and the first load subtraction coefficient Ka and / or Influence of beam hardening due to change in tube voltage (radiation energy) that sets second load subtraction coefficient Kb Even if different for each shooting, so that stable energy subtraction results, adjust the load subtraction factor.

  The embodiment of the present invention is not limited to the above embodiment. For example, the image processing installed in the imaging console using FPD is illustrated, but the image processing workstation connected to the imaging system via the network and the image archiving communication system (PACS: Picture Archiving and Communication System) It is also possible to apply to energy subtraction processing for image data transferred to the above.

  In addition, the imaging modality is not limited to FPD, and can be applied to a two-shot energy sub using a CR system. Also, in a CR-DR mixed environment where CR and FPD are connected to the shooting console at the same time, the energy sub coefficient parameter set for CR (1 shot method) and the energy sub for DR (2 shot) The coefficient parameter set may be switched according to the captured modality.

  Furthermore, by performing smoothing of the soft part image SP generated by the subtraction process and subtracting the soft part image SP in which noise is suppressed from the high energy image HP, a bone part image BP in which noise is suppressed can be obtained. .

  Further, by subtracting the bone image BP in which noise is suppressed from the high energy image HP or an addition image obtained by adding the high energy image HP and the low energy image LP, a soft part image SP in which noise is suppressed can be obtained. It is also possible to suppress noise in stages by repeating the noise suppression process and the subtraction process from the diagnostic image.

  In the above embodiment, the subtraction processing unit 31 exemplifies a case where the soft part image SP and the bone part image BP when the chest is photographed is generated as the subtraction image Psub, but the human breast photographed by the mammography apparatus. An image in which a mammary gland is emphasized and an image in which a malignant tumor is emphasized may be generated as a subtraction image Psub.

The block diagram which shows preferable embodiment of the energy subtraction processing apparatus of this invention The graph which shows an example of the 1st load subtraction table memorize | stored in the table database of FIG. The graph which shows an example of the 2nd load subtraction table memorize | stored in the table database of FIG. A graph showing how radiation is attenuated in the bone or soft part of the subject Graph showing the state where radiation energy is attenuated by beam hardening Graph showing the influence of photoelectric effect and Compton effect in beam hardening on the intensity of radiation energy A graph showing an example of the energy spectrum when low-energy radiation and high-energy radiation are irradiated to a region where the thickness of the subject is locally small A graph showing the energy spectrum when low-energy radiation and high-energy radiation are irradiated to a region where the thickness of the subject is locally large The graph which shows an example of the joint histogram of the low energy image and the high energy image in the double exposure method The graph which shows an example of the joint histogram of the low energy image and the high energy image in the single exposure method The flowchart which shows preferable embodiment of the energy subtraction processing method of this invention. The block diagram which shows 2nd Embodiment of the energy subtraction processing apparatus of this invention. A graph showing an example of a histogram of a low energy image and a high energy image when shooting a subject with a large thickness A graph showing an example of a histogram of a low energy image and a high energy image when a subject with a small thickness is photographed The graph which shows a mode that the 1st load subtraction coefficient set using the 1st load subtraction table is adjusted in the parameter setting means of FIG. The block diagram which shows 3rd Embodiment of the energy subtraction processing apparatus of this invention. The graph which shows an example of the 1st load subtraction table when it is tube voltage = 60keV at the time of acquisition of the low energy image memorize | stored in the table database of FIG. The graph which shows an example of the 1st load subtraction table when it is tube voltage = 70 keV at the time of acquisition of the low energy image memorize | stored in the table database of FIG. The graph which shows an example of the 1st load subtraction table when it is tube voltage = 80 keV at the time of acquisition of the low energy image memorize | stored in the table database of FIG. Graph showing the change of the energy spectrum with respect to the change of the tube voltage value when acquiring a high energy image Graph showing change in energy spectrum with respect to change in tube voltage when acquiring low energy images

Explanation of symbols

1, 100, 200 Energy subtraction processing device 10 Image acquisition means 15 Gradation processing means 20 Positioning means 30, 130, 140 Image processing means 31 Subtraction processing means 32, 132, 232 Parameter setting means 40 Image adjustment means 140 Thickness detection means 232 Parameter setting means BP Bone part image SP Soft part image GpH Scale factor value GpL of high energy image Scale factor value HP of low energy image High energy image LP Low energy image Ka First load subtraction coefficient Kb Second load subtraction coefficient KT1, KT11 ~ KT13 First load subtraction table KT2 Load subtraction table

Claims (13)

Image acquisition means for acquiring a high energy image and a low energy image when a subject is irradiated with radiation of different energy from a radiation source, and
The pixel density value of the high energy image and the first load subtraction coefficient multiplied by the high energy image are associated with each other such that the first load subtraction coefficient becomes non-linearly smaller as the pixel density value of the high energy image becomes higher. a table database storing first load subtraction table has,
Depending on the density value of each pixel of the high-energy image image acquired by the image acquiring means, the parameter setting said setting said first load subtraction coefficient for each pixel using the first load subtraction table Means,
Based on the first load subtraction coefficient set by the parameter setting means and the second load subtraction coefficient that is a constant predetermined value regardless of the pixel density value of the low energy image multiplied by the low energy image, An energy subtraction processing apparatus comprising: an image processing unit that performs energy subtraction processing using a high energy image and the low energy image. Image acquisition means for acquiring a high energy image and a low energy image when a subject is irradiated with radiation of different energy from a radiation source, and
Before SL corresponding to the second load subtraction coefficient and said second load subtraction coefficient higher pixel density value increases the low-energy image the pixel density value is multiplied by the low-energy image of low energy image increases nonlinearly A table database storing the attached second load subtraction table;
Depending on the density value of each pixel before Symbol low energy image obtained by the image obtaining means, the parameter setting for setting a pre-Symbol second load subtraction coefficient for each said pixel using the previous SL second load subtraction table Means,
Based on the previous SL second load subtraction coefficient set by the high energy first load subtraction factor is a predetermined value constant irrespective of the pixel density value of the high energy image to be multiplied to the image and the parameter setting means, An energy subtraction processing apparatus comprising: an image processing unit that performs energy subtraction processing using the high energy image and the low energy image. Image acquisition means for acquiring a high energy image and a low energy image when a subject is irradiated with radiation of different energy from a radiation source, and
The pixel density value of the high energy image and the first load subtraction coefficient multiplied by the high energy image are associated with each other such that the first load subtraction coefficient becomes non-linearly smaller as the pixel density value of the high energy image becomes higher. It was first load subtraction table, and the low energy pixel density value and the low energy image second load subtraction coefficient to be multiplied by the said second load subtraction coefficient higher pixel density value increases the low energy image of the image A table database storing a second load subtraction table associated so that becomes nonlinearly large;
Depending on the density value of each pixel of the high energy image and before Symbol low energy image obtained by the image acquisition means, using said first load subtraction table and before Symbol second load subtraction table and parameter setting means for setting the first load subtraction factor and before Symbol second load subtraction factor for each pixel,
Based on the first load subtraction factor and before Symbol second load subtraction coefficient set by said parameter setting means, image processing means for performing energy subtraction processing using said low-energy image and the high energy image An energy subtraction processing apparatus comprising: A thickness detecting means for detecting the thickness of the subject;
The first load subtraction coefficient and / or the parameter setting means set using the first load subtraction table and / or the second load subtraction table based on the thickness of the subject detected by the thickness detection means. The energy subtraction processing device according to any one of claims 1 to 3, wherein the second load subtraction coefficient is adjusted. 5. The energy subtraction processing apparatus according to claim 4, wherein the parameter setting means adjusts the absolute value of the first load subtraction coefficient to a greater extent as the thickness of the subject increases. The thickness detecting means calculates a signal distribution width of the high energy image and the low energy image, and the parameter setting means calculates the high energy image and the low energy image calculated by the thickness detecting means. The first load subtraction coefficient and / or the second load subtraction coefficient set using the first load subtraction table and / or the second load subtraction table is adjusted based on the ratio of the signal distribution widths. energy subtraction processing apparatus according to any one of claims 1-5, characterized in that. Tube voltage detecting means for detecting a tube voltage applied to the radiation source at the time of acquiring the low energy image;
The table database stores a plurality of the first load subtraction tables and / or a plurality of the second load subtraction tables for each tube voltage at the time of acquiring the low energy image,
Based on the tube voltage detected by the tube voltage detection means, the parameter setting means may select any one of the first loads from the plurality of first load subtraction tables and / or the plurality of second load subtraction tables. select subtraction table and / or the second load subtraction table, claim 1, wherein 6 of that used to set the first load subtraction factor and / or the second load subtraction factor 1 The energy subtraction processing device according to item. Acquire a high energy image and a low energy image when the subject is irradiated with radiation of different energy from the radiation source,
The pixel density value of the high energy image and the first load subtraction coefficient multiplied by the high energy image are associated with each other such that the first load subtraction coefficient becomes non-linearly smaller as the pixel density value of the high energy image becomes higher. by using the first load subtraction table, sets the number of first load subtraction coefficient for each respective pixel in accordance with the density value of each pixel of the high-energy image image acquired,
Based on the set first load subtraction coefficient and the second load subtraction coefficient that is a constant predetermined value regardless of the pixel density value of the low energy image multiplied by the low energy image, the high energy image and the low energy image An energy subtraction processing method characterized by performing energy subtraction processing using an energy image. Acquire a high energy image and a low energy image when the subject is irradiated with radiation of different energy from the radiation source,
Before SL corresponding to the pixel density value and the low energy image second load subtraction coefficient to be multiplied by the said second load subtraction coefficient higher pixel density value increases the low energy image of the low energy image increases nonlinearly with attached has been second load subtraction table, set the previous SL second load subtraction coefficient for each respective pixel in accordance with the density value of each pixel before Symbol low energy image obtained,
Based on the first load subtraction coefficient and the set second load subtraction coefficient that are constant predetermined values regardless of the pixel density value of the high energy image multiplied by the high energy image , the high energy image and the low energy image An energy subtraction processing method characterized by performing energy subtraction processing using an energy image. Acquire a high energy image and a low energy image when the subject is irradiated with radiation of different energy from the radiation source,
The pixel density value of the high energy image and the first load subtraction coefficient multiplied by the high energy image are associated with each other such that the first load subtraction coefficient becomes non-linearly smaller as the pixel density value of the high energy image becomes higher. It was first load subtraction table, and the low energy pixel density value and the low energy image second load subtraction coefficient to be multiplied by the said second load subtraction coefficient higher pixel density value increases the low energy image of the image There using the second load subtraction table that is associated so as to increase nonlinearly, the first for each respective pixel in accordance with the density value of each pixel of the acquired high energy image and before Symbol low energy image 1 set load subtraction factor and before Symbol second load subtraction factor,
Based on the set first load subtraction factor and before Symbol second load subtraction factor, the energy subtraction processing method, which comprises carrying out the energy subtraction processing using said low-energy image and the high energy image. On the computer,
Acquire a high energy image and a low energy image when the subject is irradiated with radiation of different energy from the radiation source,
The pixel density value of the high energy image and the first load subtraction coefficient multiplied by the high energy image are associated with each other such that the first load subtraction coefficient becomes non-linearly smaller as the pixel density value of the high energy image becomes higher. by using the first load subtraction table, sets the number of first load subtraction coefficient for each respective pixel in accordance with the density value of each pixel of the high-energy image image acquired,
Based on the set first load subtraction coefficient and the second load subtraction coefficient that is a constant predetermined value regardless of the pixel density value of the low energy image multiplied by the low energy image, the high energy image and the low energy image An energy subtraction processing program for executing energy subtraction processing using energy images. On the computer,
Acquire a high energy image and a low energy image when the subject is irradiated with radiation of different energy from the radiation source,
Before SL corresponding to the pixel density value and the low energy image second load subtraction coefficient to be multiplied by the said second load subtraction coefficient higher pixel density value increases the low energy image of the low energy image increases nonlinearly with attached has been second load subtraction table, set the previous SL second load subtraction coefficient for each respective pixel in accordance with the density value of each pixel before Symbol low energy image obtained,
Based on the first load subtraction coefficient and the set second load subtraction coefficient that are constant predetermined values regardless of the pixel density value of the high energy image multiplied by the high energy image , the high energy image and the low energy image An energy subtraction processing program for executing energy subtraction processing using energy images. On the computer,
Acquire a high energy image and a low energy image when the subject is irradiated with radiation of different energy from the radiation source,
The pixel density value of the high energy image and the first load subtraction coefficient multiplied by the high energy image are associated with each other such that the first load subtraction coefficient becomes non-linearly smaller as the pixel density value of the high energy image becomes higher. It was first load subtraction table, and the low energy pixel density value and the low energy image second load subtraction coefficient to be multiplied by the said second load subtraction coefficient higher pixel density value increases the low energy image of the image There using the second load subtraction table that is associated so as to increase nonlinearly, the first for each respective pixel in accordance with the density value of each pixel of the acquired high energy image and before Symbol low energy image 1 set load subtraction factor and before Symbol second load subtraction factor,
Based on the set first load subtraction factor and before Symbol second load subtraction factor, the high-energy image and the low energy image and the energy subtraction processing program for executing the performing the energy subtraction processing using .