JP5535528B2 - X-ray CT system - Google Patents

Description

  The present invention relates to an X-ray CT (Computed Tomography) apparatus that performs dual energy imaging.

  A method of generating an image representing a subject imaged using X-rays having desired X-ray energy based on projection data (data) obtained by performing dual energy imaging of the subject with an X-ray CT apparatus. Are known.

  For example, while the X-ray tube voltage is a first X-ray tube voltage that is relatively high, the subject is irradiated with X-rays to collect projection data based on the first X-ray tube voltage, and the X-ray tube voltage The subject is irradiated with X-rays when the second X-ray tube voltage is relatively low, and projection data based on the second X-ray tube voltage is collected. Then, based on the projection data based on the first X-ray tube voltage and the projection data based on the second X-ray tube voltage, image reconstruction processing, weighted addition / subtraction processing between the projection data or between the images, and the like are performed. An image representing a subject imaged using X-rays having linear energy (hereinafter referred to as an image with desired X-ray energy) is generated. (For example, see Patent Document 1, FIG. 4, Patent Document 2, etc.)

JP 2009-095405 A US2009 / 0052612

  By the way, when performing dual energy imaging, projection is performed by using at least a first X-ray tube current and a second X-ray tube voltage at the time of collecting projection data by using the first X-ray tube voltage. It is necessary to set a second X-ray tube current for collecting data.

  On the other hand, since the image based on the desired X-ray energy is generated using the projection data based on the first X-ray tube voltage and the projection data based on the second X-ray tube voltage, the image noise level (image noise) in the image is determined. level) depends on the noise, that is, the signal-noise ratio, in the projection data based on the first X-ray tube voltage and the projection data based on the second X-ray tube voltage. The SN ratios in the projection data based on the first X-ray tube voltage and the projection data based on the second X-ray tube voltage depend on the first and second X-ray tube currents. That is, the image noise level in the image with the desired X-ray energy depends on the first and second X-ray tube currents set as the imaging conditions. Further, in generating the image with the desired X-ray energy, the contribution ratio between the projection data based on the first X-ray tube voltage and the projection data based on the second X-ray tube voltage is expressed as “desired X-rays”. It depends on “energy”. Therefore, the relationship between the image noise level in the image and the first and second X-ray tube currents differs depending on the “desired X-ray energy”.

  However, it is usually difficult for the operator to intuitively estimate the relationship between the image noise level in the image based on the desired X-ray energy to be generated and the imaging conditions for dual energy imaging.

  Therefore, even if the operator tries to obtain an image based on the desired X-ray energy with an optimum image quality, that is, with a minimum image noise level, it is not possible to easily specify the imaging conditions for dual energy imaging.

  In view of the above circumstances, the present invention facilitates imaging conditions for dual energy imaging for generating an image representing a subject imaged using X-rays having desired X-ray energy at a desired image noise level. It is an object of the present invention to provide an X-ray CT apparatus that can be specified as follows.

  In a first aspect, the present invention includes an X-ray tube, images a subject with dual energy, first projection data based on a first X-ray tube voltage, and the first X-ray tube. Data collection means for collecting second projection data based on a second X-ray tube voltage different from the voltage, and X-rays having a single X-ray energy using the first and second projection data An X-ray CT apparatus including an image generation unit that generates a predetermined image representing a subject imaged by using an input reception unit that receives input of X-ray energy information that specifies the X-ray energy; Based on the input X-ray energy information, the X-ray energy of the image representing the subject imaged using X-rays having a single X-ray energy and the first and second projection data are collected. And each tube current time product when Shooting condition specifying means for specifying shooting conditions for dual energy shooting such that the predetermined image is generated at a desired image noise level using a relationship with an image noise level of the predetermined image; An X-ray CT apparatus is provided.

  Here, the “tube current time product” is a product of the X-ray tube current at the time of projection data acquisition and the X-ray irradiation time or data acquisition time, and the X-ray irradiation dose at the time of projection data acquisition, that is, X-ray photons. It is an indicator proportional to the number.

  The “predetermined image” is one of dual energy images obtained by dual energy imaging, and is a so-called monochromatic image.

  The “image noise level” can be specified using, for example, an image SD value (standard deviation of noise).

  In a second aspect, the present invention provides the X-ray CT apparatus according to the first aspect, wherein the X-ray energy information is an X-ray tube voltage or effective X-ray energy.

  In a third aspect, according to the present invention, the imaging condition specifying unit includes a storage unit that stores the relationship, and using the relationship, the condition relating to each tube current time product is used as the imaging condition. The X-ray CT apparatus according to the first aspect or the second aspect to be specified is provided.

  In a fourth aspect, the present invention relates to a case in which the imaging condition specifying unit collects the first X-ray tube current to be set when collecting the first projection data and the second projection data. An X-ray CT apparatus according to the third aspect is provided that specifies a ratio with a second X-ray tube current to be set as the imaging condition.

  In a fifth aspect, the present invention provides that the input receiving unit further receives an input of exposure dose information for specifying the exposure dose of the subject by the dual energy imaging, and the imaging condition specifying unit The first and second X-ray tube currents are used as the imaging conditions based on the ratio between the first X-ray tube current and the second X-ray tube current that has been input and the input dose information. The X-ray CT apparatus according to the fourth aspect to be specified is provided.

  As “exposure dose information”, for example, a CTDI (CT Dose Index) value or the like can be considered.

  In a sixth aspect, the present invention relates to a scan for a predetermined view angle that is not less than π + fan angle and not more than 2π by the first X-ray tube voltage. The X-ray CT apparatus according to the fourth aspect or the fifth aspect, wherein scanning for the predetermined view angle by the second X-ray tube voltage is performed.

  In a seventh aspect, the present invention is directed to the data collection unit, wherein the X-ray tube voltage is scanned while switching between the first X-ray tube voltage and the second X-ray tube voltage in units of one or more views. And the ratio of the data collection time to be set when the first projection data is collected and the data collection time to be set when the second projection data is collected by the imaging condition specifying means An X-ray CT apparatus according to the third aspect specified as the imaging condition is provided.

  Here, the “multiple views” are, for example, 10 views or less.

  In an eighth aspect, the present invention provides any one of the first to seventh aspects, further comprising a correction unit that corrects the specified imaging condition based on the size of the subject. An X-ray CT apparatus according to one aspect is provided.

  In a ninth aspect, the present invention provides the image generating means, wherein the first projection data is subjected to image reconstruction processing to reconstruct the first image, and the second projection data is subjected to image reconstruction processing. And reconstructing means for reconstructing the second image, weighting and subtracting the first image and the second image with a first weighting to obtain a third image, and the first image A weighted subtracting means for obtaining a fourth image by weighting and subtracting the image and the second image with a second weighting different from the first weighting; and the third image and the fourth image. There is provided an X-ray CT apparatus according to any one of the first to eighth aspects, comprising weighted addition means for performing weighted addition processing to generate the predetermined image.

  Here, the “third image” and the “fourth image” are images in which a component representing the first substance is suppressed and a component representing the second substance different from the first substance is emphasized, An image in which the component representing the second substance is suppressed and the component representing the first substance is emphasized can be considered. These images are also called substance enhancement images, substance density images, substance separation images, or substance discrimination images.

  In a tenth aspect, according to the present invention, the image generation means obtains third projection data by performing weighted subtraction processing on the first projection data and the second projection data by a first weighting, Weighted subtracting means for obtaining fourth projection data by weighting and subtracting the first projection data and the second projection data with a second weight different from the first weight; and the third projection Weight addition means for obtaining fifth projection data by weighted addition processing of data and the fourth projection data, and reconstruction for reconstructing the predetermined image by image reconstruction processing of the fifth projection data And an X-ray CT apparatus according to any one of the first to eighth aspects.

  The weighted addition process and the weighted subtraction process in the ninth aspect and the tenth aspect may be an addition process and a subtraction process based on higher-order weighting.

  In an eleventh aspect, the present invention provides the X-ray CT apparatus according to any one of the first to tenth aspects, wherein the desired image noise level is a minimum image noise level.

  According to the present invention, the imaging condition specifying means, based on the input X-ray energy information, the X-ray energy of an image representing a subject imaged using X-rays having a single X-ray energy; A single X specified by the X-ray energy information using the relationship between the respective tube current time products when collecting the first and second projection data and the image noise level of the predetermined image. Since a predetermined image representing a subject imaged using X-rays having line energy is generated at a desired image noise level, the imaging condition of dual energy imaging is specified. It is possible to easily specify shooting conditions of dual energy shooting for generating at the image noise level.

It is a figure which shows the structure of the X-ray CT apparatus which concerns on 1st embodiment. It is a figure for demonstrating the 1st method of dual energy imaging | photography. It is a figure for demonstrating the 2nd method of dual energy imaging | photography. It is a flowchart (flowchart) which shows an example of an image generation process. It is a figure which shows an image generation process notionally. It is a figure which shows the spectrum (energy distribution) of the X-rays which generate | occur | produce from an X-ray tube. It is a figure which shows the relationship between the X-ray photon energy (photon energy) and X-ray absorption coefficient in a predetermined substance. It is a flowchart which shows another example of an image generation process. It is a flowchart which shows the flow of a process in the X-ray CT apparatus of 1st embodiment. It is a figure which shows an example of the imaging condition setting screen displayed on a monitor (monitor). It is a flowchart which shows an imaging condition setting process. It is a figure which shows the relationship between the effective X-ray energy of a production | generation image, and an image noise level. It is a figure which shows an example of the relationship between the effective X-ray energy of a production | generation image, and the imaging conditions that an image noise level becomes the minimum. It is a figure which shows an example of the imaging condition setting screen by 2nd embodiment. It is a flowchart which shows the imaging condition setting process by 2nd embodiment. It is a figure which shows another example of the imaging condition setting screen displayed on a monitor. It is a figure which shows an example of the imaging condition setting screen by 3rd embodiment. It is a flowchart which shows the imaging | photography condition setting process by 3rd embodiment. It is a figure which shows notionally projection data collection when the ratio of the data collection time per view is changed. It is a figure which shows a mode that the relationship between the effective X-ray energy of a production | generation image and an image noise level changes according to the magnitude | size of a subject.

  Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited thereby.

(First embodiment)
FIG. 1 is a diagram schematically showing the configuration of the X-ray CT apparatus according to the first embodiment.

  The X-ray CT apparatus 100 includes an operation console 1, an imaging table 10, and a scanning gantry 20.

  The operation console 1 is acquired by an input device 2 that receives input from an operator, a central processing unit 3 that performs control of each unit for performing dual energy imaging, data processing for generating an image, and the like, and a scanning gantry 20. A data collection buffer (buffer) 5 that collects the data, a monitor 6 that displays an image, and a storage device 7 that stores a program, data, and the like.

  The imaging table 10 includes a cradle 12 on which the subject 40 is placed and put into and out of the opening B of the scanning gantry 20. The cradle 12 is moved up and down and horizontally moved by a motor built in the imaging table 10. Here, the body axis direction of the subject 40, that is, the linear movement direction of the cradle 12 is the z direction, the vertical direction is the y direction, and the horizontal direction perpendicular to the z direction and the y direction is the x direction.

  The scanning gantry 20 includes a rotating unit 15 and a main body 20a that rotatably supports the rotating unit 15. The rotating unit 15 includes an X-ray tube 21, an X-ray controller (controller) 22 that controls the X-ray tube 21, and an X-ray beam (beam) Xb generated from the X-ray tube 21 by collimating and shaping. Collimator 23, X-ray detector 24 for detecting the X-ray beam Xb transmitted through the subject 40, and DAS (Data Acquisition System) for converting the output of the X-ray detector 24 into projection data and collecting it (Also referred to as a data collection device) 25 and a rotation unit controller 26 for controlling the X-ray controller 22, the collimator 23, and the DAS 25 are mounted. The main body 20 a includes a control controller 29 that communicates control signals and the like with the operation console 1 and the imaging table 10. The rotating part 15 and the main body part 20 a are electrically connected via a slip ring 30.

  The scanning gantry 20 is an example of data collection means in the present invention. The input device 2 is an example of an input receiving unit in the present invention. The central processing unit 3 is an example of an image generation unit, a photographing condition specifying unit, a reconstruction unit, a weighting subtraction unit, a weighting addition unit, and a correction unit in the present invention. By executing a predetermined program, the central processing unit 3 It functions as a means.

  Here, in order to facilitate understanding of the processing related to the specification of imaging conditions for dual energy imaging, which is a feature of the present invention, dual energy imaging and image generation processing based on projection data obtained by dual energy imaging explain.

  First, dual energy imaging will be described.

  2 and 3 are diagrams for explaining a method of dual energy imaging.

  For dual energy imaging, for example, as shown in FIG. 2, the X-ray tube voltage of the X-ray tube 21 is set to a relatively high first X-ray tube voltage HV and a relatively low second X-ray tube voltage LV. In addition, a method of collecting projection data by detecting transmission X-rays of the subject 40 for a predetermined view angle while switching at high speed in units of one or several views. The predetermined view angle is set to, for example, a π + X-ray beam fan angle α [rad] corresponding to a half scan to 2π [rad] corresponding to a full scan.

  Further, for example, as shown in FIG. 3, the X-ray tube voltage of the X-ray tube 21 is kept at one X-ray tube voltage (for example, the first X-ray tube voltage HV) and transmitted through the subject 40 by a predetermined view angle. X-rays are detected, and then the X-ray tube voltage of the X-ray tube 21 is kept at the other X-ray tube voltage (for example, the second X-ray tube voltage LV) and transmitted X-rays of the subject 40 by a predetermined view angle. Is used to collect projection data.

  Thereby, the first projection data PHV based on the first X-ray tube voltage HV and the second projection data PLV based on the second X-ray tube voltage LV are collected via the slip ring 30. However, in the case of the former method, the first and second projection data PHV and PLV cannot collect the projection data of the view by the other X-ray tube voltage switched to each other. Therefore, the projection data that could not be collected is obtained by performing weighted addition processing on the projection data of the view adjacent to the view or filtering in the view direction.

In this example, the first and second X-ray tube voltages HV and LV are 140 [kV] and 80 [kV], respectively. The first and second projection data PHV and PLV are, for example, a half scan (half angle) in which a view angle range is a fan angle α (rad) of a π + X-ray beam (X-ray beam). scan) or full scan (full) (view angle range 2π [rad])
scan) projection data.

  Next, image generation processing will be described. In this image generation process, a subject imaged using X-rays having desired X-ray energy based on projection data obtained by dual energy imaging, that is, two types of projection data having different X-ray tube voltages. An image representing is generated. In this example, a desired X-ray tube voltage is determined, and an image (mono image) representing a subject imaged using monochromatic X-rays having the same effective X-ray energy as the X-ray effective X-ray energy by the X-ray tube voltage. Called a chromatic image). Here, for simplicity, the desired X-ray tube voltage is referred to as a “target X-ray tube voltage”, and the image is referred to as a “target X-ray tube voltage image”.

  FIG. 4 is a flowchart illustrating an example of image generation processing. FIG. 5 is a diagram conceptually showing the image generation processing.

  In step Z1, as shown in FIG. 4, the first projection data PHV is subjected to image reconstruction processing to reconstruct the first X-ray tube voltage image GHV (first image), and the second The second X-ray tube voltage image GLV (second image) is reconstructed by performing image reconstruction processing on the projection data PLV.

  Specifically, first, predetermined pre-processing including logarithmic conversion, correction of radiation quality, correction of sensitivity of the X-ray detector, and the like is performed on the first and second projection data PHV and PLV. Next, a predetermined reconstruction function is superimposed on the preprocessed first and second projection data PHV and PLV. Thereafter, the first and second projection data PHV and PLV on which the reconstruction function is superimposed are respectively backprojected to obtain first and second X-ray tube voltage images GHV and GLV.

  In step Z2, as shown in FIG. 4, the first X-ray tube voltage image GHV and the second X-ray tube voltage image GLV are weighted and subtracted by the first weighting to obtain the density distribution of the first substance. To obtain a first material density image (third image). In addition, the first X-ray tube voltage image GHV and the second X-ray tube voltage image GLV are weighted and subtracted by the second weighting to obtain a second substance density image ( A fourth image) is obtained. In this example, the first substance is water, the second substance is iodine, and a first substance density image W representing the density distribution of water and a second substance density image Io representing the density distribution of iodine are obtained. .

  The first and second material density images W and Io can be obtained by performing weighted subtraction processing according to the following mathematical formula, for example.

  Here, kw is a conversion coefficient for expressing the pixel value W (x, y) of the first substance density image by the density [mg / ml] of water, and kio is the pixel value Io (x of the second substance density image. , y) is a conversion coefficient to express iodine density [mg / ml], Rio is the dual energy ratio of iodine in the first X-ray tube voltage HV and the second X-ray tube voltage LV, and Rw is the first This is the dual energy ratio of water in the X-ray tube voltage HV and the second X-ray tube voltage LV.

  In Equation 1 representing the weighted subtraction process for obtaining the first material density image W (x, y), the weighting factor for multiplying the pixel value GLV (x, y) of the second X-ray tube voltage image is kw. Yes, the weighting factor by which the pixel value GHV (x, y) of the first X-ray tube voltage image is multiplied is kW · Rio. In Equation 2 representing the weighted subtraction process for obtaining the second material density image Io (x, y), the weighting coefficient to be multiplied by the pixel value GLV (x, y) of the second X-ray tube voltage image is kio, and the weighting factor by which the pixel value of the first X-ray tube voltage image GHV (x, y) is multiplied is kio · Rw.

  The dual energy ratio Rio is a pixel value (CT value) GCio, LV (x corresponding to iodine on the image GCLV (x, y) obtained when the X-ray is taken with the second X-ray tube voltage LV. , y) is divided by a pixel value GCio, HV corresponding to iodine on the image GCHV (x, y) obtained when X-rays are taken with the first X-ray tube voltage HV ( Pixel value ratio). Further, the dual energy ratio Rw is obtained by converting the pixel value GCw, LV (x, y) corresponding to water on the image GCLV (x, y) to the pixel value GCw, LV corresponding to water on the image GCHV (x, y). A value obtained by dividing by HV (x, y).

  Details of this process will be described below.

  FIG. 6 is a diagram showing a spectrum of X-rays generated from the X-ray tube. In this figure, SP1 is an X-ray spectrum based on a first X-ray tube voltage HV (eg 140 [kV]), and SP2 is an X-ray spectrum based on a second X-ray tube voltage LV (eg 80 [kV]). Show. FIG. 7 is a diagram showing the relationship between the X-ray photon energy and the X-ray absorption coefficient in a predetermined substance. In this figure, μa represents the X-ray absorption coefficient of the substance a, and μb represents the X-ray absorption coefficient of the substance b.

  As shown in FIG. 6, the X-ray spectrum varies depending on the X-ray tube voltage when the X-ray is generated. Further, as shown in FIG. 7, the X-ray absorption coefficient of the substance changes according to the X-ray photon energy, and the change curve varies depending on the type of element constituting the substance, the combination thereof, the ratio, or the like.

  On the other hand, the projection data of the subject or an image reconstructed based on the projection data can be approximately expressed by the density distributions of two different types of substances and their X-ray absorption coefficients. Based on this approximate model, the first substance density image W and the second substance density image Io can be obtained. That is, by subjecting the first X-ray tube voltage image GHV and the second X-ray tube voltage image GLV to weighted subtraction processing so that the pixel value of the pixel corresponding to water as the first substance becomes zero, The component of water as the first substance is suppressed, and a second substance density image Io representing the density distribution of iodine as the second substance is obtained. Similarly, the first X-ray tube voltage image GHV and the second X-ray tube voltage image GLV are weighted and subtracted so that the pixel value of the pixel corresponding to iodine as the second substance becomes zero. The component of iodine which is the second substance is suppressed, and a first substance density image W representing the density distribution of water which is the first substance is obtained.

  Note that the pixel values corresponding to the first and second substances in the first and second X-ray tube voltage images GHV and GLV can be estimated based on imaging conditions of dual energy imaging. Therefore, the weight to be used for the weighted subtraction process can be specified from the ratio of these estimated pixel values.

  In step Z3, as shown in FIG. 4, the first substance density image W and the second substance density image Io are weighted and added and photographed using X-rays with a predetermined X-ray tube voltage NV. An image GNV representing the subject is obtained.

  The image GNV can be obtained, for example, by performing a weighted addition process according to the following formula.

  Here, keV1 is the effective X-ray energy of the X-ray by the X-ray tube voltage NV, μw (keV1) is the X-ray absorption coefficient of water at the effective X-ray energy keV1, and μio (keV1) is the iodine of the effective X-ray energy keV1. An X-ray absorption coefficient, kc, is a conversion coefficient for converting the pixel value of the image by the X-ray tube voltage NV into a CT value. As is well known, the CT value is a value indicating the degree of X-ray absorption of a substance, the CT value of air is represented by -1000 [HU], and the CT value of water is represented by 0 [HU].

  In Formula 5 representing the weighted addition process for obtaining the image GNV, the weighting factor to be multiplied with the first substance density image W (x, y) is kc · μw (keV1), and the second substance density image Io ( The weighting coefficient multiplied by x, y) is kc · μio (keV1).

  The above is an example of the image generation process, but the image generation process is not limited to the above process, and for example, the following process may be used.

  FIG. 8 is a flowchart illustrating another example of the image generation process.

  In step Z11, the first projection data PHV and the second projection data PLV are weighted and subtracted by the first weighting, and the first material density image representing the density distribution of the first material is obtained. 3 projection data PW is obtained. Further, a second material density image representing a density distribution of a second substance different from the first substance by performing a weighted subtraction process on the first projection data PHV and the second projection data PLV with a second weighting. To obtain fourth projection data PIO.

  In step Z12, the third projection data PW and the fourth projection data PIO are weighted and added, and based on an image GNV representing the subject imaged using X-rays with a predetermined X-ray tube voltage NV. The fifth projection data PNV is obtained.

  In step Z13, the fifth projection data PNV is subjected to image reconstruction processing to reconstruct an image GNV representing the subject imaged using X-rays with a predetermined X-ray tube voltage NV.

  Hereafter, the flow of processing in the X-ray CT apparatus of the first embodiment will be described.

  FIG. 9 is a flowchart showing the flow of processing in the X-ray CT apparatus of the first embodiment.

  In step S1, shooting conditions for dual energy shooting are set (hereinafter, this processing is referred to as shooting condition setting processing). Details of the photographing condition setting process will be described later with reference to FIGS. 10 and 11.

  In step S2, the above-described dual energy imaging is performed according to the imaging conditions set in step S1.

  In step S3, the above-described image generation processing is performed to generate a target X-ray tube voltage image based on the projection data obtained by the dual energy imaging in step S2.

  In step S4, the image generated in step S3 is displayed on the monitor.

  Here, details of the photographing condition setting process (S1) will be described.

  The shooting condition setting process is performed on a shooting condition setting screen displayed on the monitor 6.

  FIG. 10 is a diagram illustrating an example of the shooting condition setting screen. FIG. 11 is a flowchart showing the photographing condition setting process.

  In the imaging condition setting screen 52, an “X-ray tube voltage of generated image” column 61 and an “effective X-ray energy” column 62 are displayed. The “generated image X-ray tube voltage” column 61 is a column for inputting and displaying the target X-ray tube voltage TV. The “effective X-ray energy” column 62 is a column that displays a target effective X-ray energy TE that is an effective X-ray energy of X-rays determined by the target X-ray tube voltage TV.

  In the imaging condition setting screen 52, a “high X-ray tube voltage” column 63, an “X-ray tube current” column 64, a “low X-ray tube voltage” column 65, an “X-ray tube current” column 66, and An “X-ray tube current ratio” column 67 is displayed. The “high X-ray tube voltage” column 63 is a column that displays a relatively high first X-ray tube voltage HV among X-ray tube voltages set in the dual energy imaging. In this example, the first X-ray tube voltage HV is 140 [kV]. The “X-ray tube current” column 64 is a column for inputting and displaying the X-ray tube current HI set when the X-ray tube 21 is set to the first X-ray tube voltage HV and the projection data is collected. The “low X-ray tube voltage” column 65 is a column that displays a relatively low second X-ray tube voltage LV among the X-ray tube voltages set in the dual energy imaging. In this example, the second X-ray tube voltage LV is 80 [kV]. The “X-ray tube current” column 66 is a column for inputting and displaying an X-ray tube current LI set when the X-ray tube 21 is set to the second X-ray tube voltage LV and projection data is collected. The “X-ray tube current ratio” column 67 is a column for displaying a target X-ray tube current ratio TR which is an imaging condition of dual energy imaging for obtaining a target X-ray tube voltage image TG at a desired image noise level. Here, the target X-ray tube current ratio TR is an X-ray tube current that minimizes the image noise level of the target X-ray tube voltage image TG when the exposure dose of the subject 40 by dual energy imaging is constant. It is the ratio of HI and X-ray tube current LI. Further, target X-ray tube current ratio TR = predetermined X-ray tube current ratio R as an imaging condition, and X-ray tube current ratio R = X-ray tube current LI / X-ray tube current HI.

  When a value is input to the “X-ray tube voltage of generated image” column 61 (step A11), this input value, that is, the target X-ray tube voltage TV is converted into the target effective X-ray energy TE (step A12).

  X-rays defined by a certain X-ray tube voltage can be expressed as X-rays defined by a certain effective X-ray energy. It may be more convenient to determine the X-ray conditions by effective X-ray energy. Therefore, in this example, the above conversion is performed. The converted target effective X-ray energy TE is displayed in the “effective X-ray energy” column 62. The conversion from the target X-ray tube voltage TV to the target effective X-ray energy TE can be performed using a function or a table that defines the correspondence between the X-ray tube voltage and the effective X-ray energy.

  When the conversion to the target effective X-ray energy TE is performed, the target X-ray tube current ratio TR is specified based on the target effective X-ray energy TE (step A13).

  Here, a method for specifying the target X-ray tube current ratio TR will be described.

  First, let us consider the relationship between the image noise level and the X-ray tube current.

  Since a general CT image is reconstructed based on projection data based on a single X-ray tube voltage, the image noise level of the CT image depends on the noise of the projection data based on the single X-ray tube voltage. . The noise of the projection data depends on the X-ray irradiation dose when collecting the projection data. Therefore, the image noise level of the CT image is determined by the X-ray irradiation dose.

  On the other hand, the image GNV is generated using the first projection data PHV based on the first X-ray tube voltage HV and the second projection data PLV based on the second X-ray tube voltage LV. Further, the first projection data PHV and the second projection data PLV contribute to the image GNV at a predetermined contribution rate determined by the effective X-ray energy E of the image GNV. That is, the image noise level of the image GNV depends on the noise of the first projection data PHV and the second projection data PLV and the balance of these noises.

  Therefore, under the condition that the sum of the exposure doses of the subject 40 is the same, the sum of the X-ray irradiation doses when collecting the first projection data PHV and the second projection data PLV is constant, and the image GNV The image noise level is determined by the balance of these X-ray irradiation doses.

  The X-ray irradiation dose is proportional to the so-called tube current time product [mA · s], which is the product of the X-ray tube current and the X-ray irradiation time or data acquisition time. Therefore, the balance of the above X-ray irradiation dose, that is, the balance between the X-ray irradiation dose by the first X-ray tube voltage HV and the X-ray irradiation dose by the second X-ray tube voltage LV is as follows. The ratio of the tube current time product at the time of collecting the first projection data PHV by the tube voltage HV and the tube current time product at the time of collecting the second projection data PLV by the second X-ray tube voltage LV (hereinafter referred to as tube current time). Product ratio).

  From the above, it can be seen that the image noise level of the image GNV is determined by the tube current time product ratio if the exposure dose of the subject 40 is constant. When the data acquisition time at the first X-ray tube voltage HV and the data acquisition time at the second X-ray tube voltage LV are the same, the image noise level of the image GNV is the X-ray tube current. It can be seen that it is determined by the ratio R.

  Here, a specific example of the relationship between the image noise level of the image GNV and the X-ray tube current ratio R will be described.

  FIG. 12 is a diagram showing the relationship between the effective X-ray energy E of the image GNV and the image noise level NL of the image GNV.

  Here, the X-ray tube current at the time of projection data collection by the X-ray tube voltage 140 [kV] which is the first X-ray tube voltage HV is a1, the data collection time per view is s1, and the second X-ray tube An X-ray tube current at the time of projection data collection with an X-ray tube voltage of 80 [kV], which is a voltage LV, is a2, and a data collection time per view is s2.

  The imaging conditions for dual energy imaging are as follows: the exposure dose of the subject 40 is set to a constant value DC, and the tube current time product ratios a1 · s1: a2 · s2 = 1 of the X-ray tube voltages 140 [kV] and 80 [kV]. 4, the relationship between the effective X-ray energy E and the image noise level NL in the image GNV is as shown by a curve N1 in FIG. Further, when the imaging condition is such that the exposure dose of the subject is a constant value DC and the tube current time product ratio is a1 · s1: a2 · s2 = 1: 2, the above relationship is as indicated by a curve N2. Further, when the imaging condition is such that the exposure dose of the subject is a constant value DC and the tube current time product ratio is a1 · s1: a2 · s2 = 1: 4, the above relationship becomes a curve N3.

  As can be seen from the curve shown in FIG. 12, the image noise level NL of the image GNV changes according to the effective X-ray energy E of the image GNV when the tube current time product ratio a1, s1: a2, s2 is constant. In addition, it can be seen that a curve having a minimum point and a convex downward is drawn at a certain effective X-ray energy. It can also be seen that this curve moves in the direction of the effective X-ray energy E when the tube current time product ratio a1 · s1: a2 · s2 is changed.

  In these phenomena, the image GNV is generated using the first projection data PHV based on the first X-ray tube voltage HV and the second projection data PLV based on the second X-ray tube voltage LV. This is because the contribution ratio of these projection data on the generation of GNV differs depending on the effective X-ray energy E of the image GNV.

  Therefore, the relationship between the effective X-ray energy E and the image noise level NL of the image GNV is obtained through experiments and simulations under a plurality of imaging conditions in which the tube current time product ratios a1 · s1: a2 · s2 are slightly different. For each effective X-ray energy E, the imaging conditions that minimize the image noise level NL of the image GNV representing the subject imaged by the X-rays of the effective X-ray energy E are set as the tube current time product ratio a 1. It can be obtained as s1: a2 · s2.

  FIG. 13 is a diagram showing an example of the relationship between the effective X-ray energy E of the image GNV and the tube current time product ratios a1, s1: a2, and s2 as imaging conditions that minimize the image noise level NL of the image GNV. is there.

  In this example, a predetermined function or table that defines such a relationship is prepared and stored in advance. Then, based on the target effective X-ray energy TE converted from the input target X-ray tube voltage TV, the image noise level NL of the target X-ray tube voltage image TG is minimized using the predetermined function or table. The tube current time product ratio a 1 · s 1: a 2 · s 2 is obtained as the imaging condition. Then, considering the ratio of the data acquisition time per view when the projection data is acquired with the first X-ray tube voltage HV (140 kV) and when the projection data is acquired with the second X-ray tube voltage LV (80 kV), The target X-ray tube current ratio TR is specified from the tube current time product ratio a1 · s1: a2 · s2 which is the obtained imaging condition. In this example, the data collection time per view is the time when projection data is collected with the first X-ray tube voltage HV (140 kV) and the time when projection data is collected with the second X-ray tube voltage LV (80 kV). The target X-ray tube current ratio TR is specified as the same.

  When the target X-ray tube current ratio TR is specified, the target X-ray tube current ratio TR is displayed in the “X-ray tube current ratio” column 65 (step A14). At this time, not only the target X-ray tube current ratio TR is simply displayed, but also the values inputted in the “X-ray tube current” column 62 and the “X-ray tube current” column 64, that is, the X-ray tube current HI and the X-rays. The tube current LI may be constrained so that these ratios become the target X-ray tube current ratio TR. For example, when a value is input to one of the “X-ray tube current” column 62 and the “X-ray tube current” column 64, the ratio of the X-ray tube current HI to the X-ray tube current LI is set to the target X. The value in the other column is automatically input so that the tube current ratio TR is obtained.

  When values are respectively input to the “X-ray tube current” column 62 and the “X-ray tube current” column 64 (step A15), the imaging conditions for dual energy imaging are set.

  According to the first embodiment, the target X-ray tube current ratio TR for obtaining the target X-ray tube voltage image TG with the minimum image noise level by simply inputting the “target X-ray tube voltage”, that is, The first X-ray tube current THI to be set at the time of collecting the first projection data PHV by the first X-ray tube voltage HV and the second projection data PLV by the second X-ray tube voltage LV should be set Since the ratio with the second X-ray tube current TLI is specified, it is possible to easily specify imaging conditions for dual energy imaging for obtaining the target X-ray tube voltage image TG with the minimum image noise level.

  In the first embodiment, in order to specify what kind of X-ray energy the image to be generated is an image representing the subject imaged, the X-ray energy is used. Although “target X-ray tube voltage” is input as X-ray energy information to be expressed, instead, “target effective X-ray energy” corresponding thereto may be input.

  In the first embodiment, in the imaging condition specifying process, the correspondence relationship between the effective X-ray energy E of the image GNV and the tube current time product ratio a1 · s1: a2 · s2 that minimizes the image noise level of the image GNV. The target X-ray tube current ratio TR, which is the imaging condition of dual energy imaging, is specified from the function and table that determine the above. Of course, the present invention is not limited to this. For example, the target X-ray tube current ratio TR is obtained from information that defines the correspondence between the X-ray tube voltage V in the image GNV and the tube current time product ratio a1 · s1: a2 · s2 that minimizes the image noise level of the image GNV. May be specified. Further, for example, the image GNV is premised on that the data collection time per view is the same between the projection data collection by the first X-ray tube voltage HV and the projection data collection by the second X-ray tube voltage LV. The correspondence relationship between the X-ray tube voltage V or the effective X-ray energy E and the ratio of the first X-ray tube current HI and the second X-ray tube current LI that minimizes the image noise level of the image GNV. The target X-ray tube current ratio TR may be specified from the information to be determined.

  In the first embodiment, the target X-ray tube current ratio TR is specified as an imaging condition for minimizing the image noise level of the target X-ray tube voltage image TG. Therefore, the method of specifying the X-ray tube current ratio as an imaging condition is a dual energy imaging in which the X-ray tube current is easily controlled, that is, a predetermined X-ray tube voltage is maintained while the X-ray tube voltage of the X-ray tube 21 is set to one X-ray tube voltage. The transmitted X-ray of the subject 40 is detected by the view angle, and then the transmitted X-ray of the subject 40 is detected by the predetermined view angle while keeping the X-ray tube voltage of the X-ray tube 21 at the other X-ray tube voltage. It is suitable for dual energy photography using the method.

(Second embodiment)
In the second embodiment, based on the target X-ray tube voltage TV and the target exposure dose TD of the subject 40 by dual energy imaging, the first target X-ray tube current THI and the 2 target X-ray tube current TLI itself is specified.

  FIG. 14 is a diagram illustrating an example of a shooting condition setting screen according to the second embodiment. FIG. 15 is a flowchart showing the imaging condition specifying process according to the second embodiment.

  In the imaging condition setting screen 53, “X-ray tube voltage of generated image” column 61, “effective X-ray energy” column 62, “high X-ray tube voltage” column 63, “X-ray tube current” column 64, “ In addition to the “Low X-ray tube voltage” column 65, the “X-ray tube current” column 66 and the “X-ray tube current ratio” column 67, an “Exposure dose” column 68 is displayed. The “exposure dose” column 68 is a column for inputting and displaying a target exposure dose TD of the subject 40 by dual energy imaging, for example, a CTDI value.

  When a value is input to the “generated image X-ray tube voltage” column 61 (step B11), the input value, that is, the target X-ray tube voltage TV is converted into the target effective X-ray energy TE (step B12). Based on the target effective X-ray energy TE, the target X-ray tube current ratio TR is specified (step B13), and the target X-ray tube current ratio TR is displayed in the "X-ray tube current ratio" column 65. (Step B14).

  Here, when a value is input to the “exposure dose” column 68 (step B15), the first target is based on the input value, that is, the target exposure dose TD and the specified target X-ray tube current ratio TR. The X-ray tube current THI and the second target X-ray tube current TLI are specified and set (step B16). The first target X-ray tube current THI and the second target X-ray tube current TLI are the X-ray tube current HI and the X-ray tube to be set in order to minimize the image noise level in the target X-ray tube voltage image TG. Current LI. These are displayed in the “X-ray tube current” column 64 and the “X-ray tube current” column 66 (step B17).

  Here, a specific example of a method for specifying the first target X-ray tube current THI and the second target X-ray tube current TLI will be described.

  First, the first exposure by X-ray when X-ray tube voltage = first X-ray tube voltage HV (= 140 [kV]), X-ray tube current = first target X-ray tube current THI [mA]. The dose D1 [mGy] can be expressed by the following equation, where d1 [mGy] is an X-ray exposure dose when the X-ray tube voltage = 140 [kV] and the X-ray tube current = 100 [mA].

  Further, the second exposure by X-ray when X-ray tube voltage = second X-ray tube voltage HV (= 80 [kV]) and X-ray tube current = second target X-ray tube current TLI [mA]. The dose D2 [mGy] can be expressed by the following equation, where d2 [mGy] is an X-ray exposure dose when the X-ray tube voltage = 80 [kV] and the X-ray tube current = 100 [mA].

  Incidentally, the relationship between the first target X-ray tube current THI and the second target X-ray tube current TLI is expressed by the following equation.

  Since the target exposure dose TD is the sum of the first exposure dose D1 and the second exposure dose D2, it can be expressed by the following equation.

  The first target X-ray tube current THI and the second target X-ray tube current TLI can be expressed as follows:

  Therefore, the first target X-ray tube current THI and the second target X-ray tube current TLI can be specified by calculating and storing the exposure doses d1 and d2 in advance.

  According to the second embodiment, the dual energy for obtaining the target X-ray tube voltage image TG with the minimum image noise level only by inputting the “target X-ray tube voltage” and the “target exposure dose”. As imaging conditions for imaging, the first X-ray tube current THI to be set at the time of collecting the first projection data PHV by the first X-ray tube voltage HV and the second projection data by the second X-ray tube voltage LV. Since the second X-ray tube current TLI to be set at the time of PLV acquisition is specified, the imaging conditions for dual energy imaging for obtaining the target X-ray tube voltage image TG with the minimum image noise level are easily specified. Can do.

  In the second embodiment, as described above, the first target X-ray tube current THI and the second target, which are imaging conditions for dual energy imaging, can be obtained simply by inputting the target X-ray tube voltage TV and the target exposure dose TD. The target X-ray tube current TLI can be specified and set.

  Therefore, the imaging condition setting screen is simplified, and for example, as shown in FIG. 16A, the imaging condition setting screen displays only the “X-ray tube voltage of generated image” column 61 and the “exposure dose” column 68. 54a may be used.

  Further, for example, as shown in FIG. 16B, an imaging condition setting screen 54 b that displays only the “generated image X-ray tube voltage” column 61 and the “generated image X-ray tube current” column 69 may be used. Good. The “generated image X-ray tube current” column 69 generates a target X-ray tube voltage image TG ′ representing the subject 40 photographed using X-rays based on the target X-ray tube voltage TV and the target X-ray tube current TI. This is a column for inputting and displaying the target X-ray tube current TI in the case of When the target X-ray tube voltage TV and the target X-ray tube current TI are input, the image noise level of an image obtained when general imaging is performed under these conditions is obtained. Then, an exposure dose is obtained such that the image noise level in the target X-ray tube voltage image TG is substantially the same as the obtained level, and this is set as the target exposure dose TD. Thereafter, the first target X-ray tube current THI and the second target X-ray tube current TLI are specified and set by the same processing as described above. According to such an example, an operator simply inputs an X-ray tube voltage and an X-ray tube current in the same manner as in general imaging, and an image representing a subject imaged using X-rays according to the conditions is displayed. It is possible to specify and set shooting conditions for dual energy shooting to obtain a minimum image noise level.

(Third embodiment)
In the third embodiment, based on the target X-ray tube voltage TV, the data acquisition time THT per view at the time of collecting the first projection data PHV by the first X-ray tube voltage HV is set as the imaging condition for dual energy imaging. The ratio Q with the data acquisition time TLT per view when the second projection data PLV is acquired by the second X-ray tube voltage LV is specified.

  FIG. 17 is a diagram illustrating an example of an imaging condition setting screen according to the third embodiment. FIG. 18 is a flowchart showing the photographing condition specifying process according to the third embodiment.

  In the imaging condition setting screen 55, “X-ray tube voltage of generated image” column 61, “effective X-ray energy” column 62, “high X-ray tube voltage” column 63, “X-ray tube current” column 64, “ In addition to the “Low X-ray tube voltage” column 65 and the “X-ray tube current” column 66, a “Data collection time ratio” column 70 is displayed. The “data collection time ratio” column 70 is a data collection time per view when collecting the first projection data PHV by the first X-ray tube voltage HV and the second projection data by the second X-ray tube voltage LV. This is a column for inputting and displaying the ratio Q to the data collection time per view at the time of PLV collection.

  When a value is input to the “generated image X-ray tube voltage” column 61 (step C11), the input value, that is, the target X-ray tube voltage TV is converted into the target effective X-ray energy TE (step C12). When values are input to the “X-ray tube current” column 64 and the “X-ray tube current” column 66, these input values, that is, the X-ray tube current HI and the X-ray tube current LI, and the target effective X-rays are obtained. Based on the energy TE, a data collection time ratio Q per view for minimizing the image noise level of the target X-ray tube voltage image TG is specified and set (step C13). Then, the data collection time ratio Q per view is displayed in the “data collection time ratio” column 70 (step C14). The data collection time ratio Q per view can be changed by re-entering a value in the “data collection time ratio” column 70.

  Here, a specific example of a method for specifying the data collection time ratio Q per view will be described.

  In this example, as in the first embodiment, as shown in FIG. 13, the tube current time product ratio a 1 ··· which is an imaging condition that minimizes the effective X-ray energy E of the image GNV and the image noise level NL of the image GNV. A predetermined function or table that defines the relationship between s1: a2 and s2 is prepared in advance. Then, based on the target effective X-ray energy TE converted from the input target X-ray tube voltage TV, the image noise level NL of the target X-ray tube voltage image TG is minimized using the predetermined function or table. The tube current time product ratio a 1 · s 1: a 2 · s 2 is obtained as the imaging condition. Then, in consideration of the X-ray tube current HI and the X-ray tube current LI, the data collection time ratio Q per view is specified from the obtained tube current time product ratio a1 · s1: a2 · s2.

  FIG. 19 shows the first X-ray tube voltage in dual energy imaging in which the X-ray tube voltage of the X-ray tube 21 is switched between the first X-ray tube voltage HV and the second X-ray tube voltage LV for each view. The ratio of the data collection time per view when collecting the first projection data PHV by HV and the data collection time per view when collecting the second projection data PLV by the second X-ray tube voltage LV is 1: It is a figure which shows notionally projection data collection when it is set to 1: 1: 4.

  According to such a third embodiment, as the imaging condition of the dual energy imaging for obtaining the target X-ray tube voltage image TG with the minimum image noise level only by inputting the “target X-ray tube voltage”, the first The data collection time per view when collecting the first projection data PHV with one X-ray tube voltage HV, and the data collection time per view when collecting the second projection data PLV with the second X-ray tube voltage LV Therefore, the dual energy imaging conditions for obtaining the target X-ray tube voltage image TG with the minimum image noise level can be easily specified.

  In the third embodiment, the data collection time ratio per view is specified as an imaging condition that minimizes the image noise level of the target X-ray tube voltage image TG. Therefore, the method of specifying the data acquisition time ratio per view as an imaging condition is dual energy imaging in which the data acquisition time per view is easier to control than the X-ray tube current, that is, the X-ray tube of the X-ray tube 21. A method of detecting transmitted X-rays of the subject 40 for a predetermined view angle while switching the voltage between the first X-ray tube voltage HV and the second X-ray tube voltage LV at high speed in units of one or several views. Suitable for dual energy photography.

(Fourth embodiment)
In the fourth embodiment, the tube current time product ratio a1 · s1: a2 · s2 that minimizes the image noise level of the target X-ray tube voltage image TG is corrected according to the size of the subject 40.

  As the subject 40 becomes larger, the slice cross-sectional area becomes larger and the degree of radiation hardening (beam hardening) in the X-rays that pass through the subject becomes larger. Since the X-ray with the relatively low second X-ray tube voltage LV contains a large amount of photons with relatively low energy, the number of photons of the transmitted X-ray increases as the degree of X-ray quality hardening increases. Decrease significantly. That is, when the subject becomes large, the SN ratio of the projection data by the second X-ray tube voltage LV is greatly reduced. On the other hand, X-rays with a relatively high first X-ray tube voltage HV contain a large amount of photons with relatively high energy, so that even if the degree of X-ray quality hardening increases, the transmitted X-rays The number of photons is not reduced as much as in the case of the second X-ray tube voltage LV. That is, even if the subject becomes large, the S / N ratio of the projection data based on the first X-ray tube voltage HV is not as small as in the case of the second X-ray tube voltage LV.

  Therefore, in the relationship between the effective X-ray energy E of the image GNV and the image noise level NL of the image GNV, as shown in FIG. 20, the effective X-ray energy that minimizes the image noise level NL when the subject becomes large. However, it moves in the direction in which the contribution ratio of the first projection data PHV by the first X-ray tube voltage HV in generating the image GNV increases, that is, in the direction in which the effective X-ray energy increases.

  Therefore, the tube current time product ratio (a2 · s2) / (a1 · s1), which is an imaging condition that minimizes the image noise level NL of the target X-ray tube voltage image TG, moves in a larger direction as the subject becomes larger. .

  Therefore, based on the target X-ray tube voltage TV or the target effective X-ray energy TE of the target X-ray tube voltage image TG, the tube current time during which the effective X-ray energy E of the image GNV and the image noise level of the image GNV are minimized. The tube current time product ratio (a2 · s2) / (a1 · s1), which is the imaging condition of dual energy imaging, is specified from a function or table that defines the correspondence relationship with the product ratio a1 · s1: a2 · s2. Then, a correction according to the size of the subject is added to this ratio to derive a more appropriate imaging condition.

  Note that the size of the subject is estimated from, for example, the height, weight, age, and sex of the subject, specified based on information input by the operator, or a scout image of the subject. Or based on The relationship between the size of the subject and the correction amount is obtained in advance by experiments or simulations.

  Further, the correction of the imaging conditions may be performed for each subject or may be performed for each slice position of the same subject.

  According to such a fourth embodiment, the imaging conditions of dual energy imaging specified based on the target X-ray tube voltage TV or the target effective X-ray energy TE of the target X-ray tube voltage image TG are set to the size of the subject. Therefore, the imaging condition for dual energy imaging that minimizes the image noise level in the generated image can be accurately specified regardless of the size of the subject.

  The above is the embodiment of the present invention, but the present invention is not limited to the above-described embodiments, and various modifications can be adopted.

  For example, in each of the above embodiments, dual energy imaging is performed using a single X-ray tube, but dual energy imaging may be performed using a plurality of X-ray tubes having different X-ray irradiation directions. Good. In this case, a relatively high first X-ray tube voltage HV is set for the first X-ray tube, and a relatively low second X-ray tube voltage LV is set for the second X-ray tube. Also good.

  In the first and second embodiments, an axial scan is assumed as a scanning method in dual energy imaging, but this may be a helical scan.

  In each of the above embodiments, the first and second substance density images are an image representing a water density distribution and an image representing an iodine density distribution, but the present invention is not limited to this combination. For example, it may be a combination of an image representing the density distribution of water and an image representing the density distribution of calcium, and a combination of an image representing the density distribution of iodine and an image representing the density distribution of calcium.

  In each of the above embodiments, the desired image noise level is set to the minimum image noise level, but the present invention is not limited to this. For example, the image noise level may be a predetermined image noise level that is slightly higher than the minimum.

  In each of the above-described embodiments, a relatively high X-ray tube voltage HV in dual energy imaging is set as the highest X-ray tube voltage that can be set, and a minimum X-ray tube that can set a relatively low X-ray tube voltage LV. It is preferable to use a voltage. As a result, the energy separation degree is increased, and an image with a good SN ratio is obtained. As a temporary measure, the relatively high X-ray tube voltage HV is set to 120 kV or more and 160 kV or less, and the relatively low X-ray tube voltage LV is set to 60 kV or more and 100 kV or less.

DESCRIPTION OF SYMBOLS 1 Operation console 2 Input device 3 Central processing unit 5 Data collection buffer 6 Monitor 7 Storage device 10 Imaging table 12 Cradle 15 Rotating unit 20 Scanning gantry 20a Main unit 21 X-ray tube 22 X-ray controller 23 Collimator 24 X-ray detector 25 DAS
26 Rotating part controller 29 Control controller 30 Slip ring 40 Subject 100 X-ray CT apparatus B Opening Xb X-ray beam

Claims (11)

An X-ray tube is provided, the subject is subjected to dual energy imaging, the first projection data based on the first X-ray tube voltage, and a second X-ray tube different from the first X-ray tube voltage Data collection means for collecting second projection data by voltage;
An X-ray CT apparatus comprising: an image generating unit that generates a predetermined image representing a subject imaged using X-rays having a single X-ray energy using the first and second projection data Because
Input receiving means for receiving input of X-ray energy information for specifying the X-ray energy;
Wherein is generated based on the input X-ray energy information, and the X-ray energy of an image representing a subject is taken using X-rays having a single X-ray energy, the first and second Dual energy such that the predetermined image is generated at a desired image noise level using the relationship between each tube current time product when the projection data is collected and the image noise level of the predetermined image An X-ray CT apparatus comprising imaging condition specifying means for specifying imaging conditions for imaging.   The X-ray CT apparatus according to claim 1, wherein the X-ray energy information is an X-ray tube voltage or effective X-ray energy.   The imaging condition specifying unit includes a storage unit that stores the relationship, and uses the relationship to specify a condition related to each of the tube current time products as the imaging condition. The X-ray CT apparatus described.   The imaging condition specifying means includes a first X-ray tube current to be set when collecting the first projection data and a second X-ray tube to be set when collecting the second projection data. The X-ray CT apparatus according to claim 3, wherein a ratio with current is specified as the imaging condition. The input receiving means further receives an input of exposure dose information for specifying an exposure dose of the subject by the dual energy imaging,
The imaging condition specifying means is configured to determine the first and second based on the ratio of the specified first X-ray tube current and the second X-ray tube current and the input dose information. The X-ray CT apparatus according to claim 4, wherein an X-ray tube current is specified as the imaging condition.   The data collection means performs a scan for a predetermined view angle that is not less than π + fan angle and not more than 2π by the first X-ray tube voltage, and a scan for the predetermined view angle by the second X-ray tube voltage. The X-ray CT apparatus according to claim 4 or 5, which is performed. The data collection means performs scanning while switching the X-ray tube voltage between the first X-ray tube voltage and the second X-ray tube voltage in units of one or more views,
The imaging condition specifying means determines a ratio between a data collection time to be set when collecting the first projection data and a data collection time to be set when collecting the second projection data. The X-ray CT apparatus according to claim 3 specified as follows.   The X-ray CT apparatus according to claim 1, further comprising a correction unit that corrects the specified imaging condition based on the size of the subject. The image generating means includes
Reconstructing means for reconstructing the first image by reconstructing the first projection data to reconstruct the first image, and reconstructing the second image by reconstructing the second projection data;
The first image and the second image are weighted and subtracted by a first weight to obtain a third image, and the first image and the second image are converted to the first weight. Are weighted subtracting means for obtaining a fourth image by weighted subtraction with different second weightings;
The X-ray according to any one of claims 1 to 8, further comprising weighted addition means for generating the predetermined image by performing weighted addition processing on the third image and the fourth image. CT device. The image generating means includes
The first projection data and the second projection data are weighted and subtracted by a first weighting to obtain third projection data, and the first projection data and the second projection data are Weighted subtracting means for obtaining fourth projection data by performing weighted subtraction with a second weight different from the first weight;
Weighting and adding means for weighting and adding the third projection data and the fourth projection data to obtain fifth projection data;
The X-ray CT apparatus according to claim 1, further comprising: a reconstruction unit configured to reconstruct the predetermined image by performing image reconstruction processing on the fifth projection data.   The X-ray CT apparatus according to claim 1, wherein the desired image noise level is a minimum image noise level.

Images