Patent Publication Number: US-11398165-B2

Title: Simulator, injection device or imaging system provided with simulator, and simulation program

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a simulator that simulates a time-dependent change in the pixel value in a tissue, an injection device or an imaging system provided with the simulator, and a simulation program. 
     Description of the Related Art 
     There has been a proposed simulator that predicts a time-dependent change in the pixel value in a tissue of a subject. International Publication No. WO2016/084373 describes a simulator including a prediction section that predicts, based on subject information, an injection protocol, and tissue information, a time-dependent change in the pixel value of each of a plurality of compartments produced by dividing a tissue along the blood flow direction. 
     SUMMARY OF THE INVENTION 
     The simulator descried in International Publication No. WO2016/084373 is not intended to use an injection protocol that maintains a target pixel value over a target duration. 
     To solve the problem described above, a simulator as an example of the present invention includes a chemical liquid information acquisition section configured to acquire an amount of contrast medium, a target value acquisition section configured to acquire a target duration for which a target pixel value is maintained, a protocol acquisition section configured to acquire a contrast medium injection protocol, and a prediction section for determining a predicted duration, the prediction section configured to simulate a time-dependent change in a pixel value in a tissue of a subject based on the injection protocol and the amount of the contrast medium, wherein the prediction section compares the predicted duration with the target duration to re-simulate, in a case where the predicted duration is shorter than the target duration, the time-dependent change in a condition where a greater amount of the contrast medium than the amount of the contrast medium used in the simulation is injected to redetermine the predicted duration, and in a case where the predicted duration is longer than the target duration, the time-dependent change in a condition where a smaller amount of the contrast medium than the amount of the contrast medium used in the simulation is injected to redetermine the predicted duration. 
     The simulator can simulate a time-course change in the pixel value in a tissue of a subject in a case where an injection protocol that maintains a target pixel value over a target duration is used. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of a simulator. 
         FIG. 2  is a view of a main screen displayed on a display section of the simulator. 
         FIG. 3  a view of an automatic optimization screen displayed on the display section of the simulator. 
         FIG. 4  is a flowchart of automatic optimization. 
         FIG. 5  is a schematic view of an injection device and an imaging system. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments for implementing the present invention will be described below in detail with reference to the drawings. It is, however, noted that the dimensions, materials, shapes, and relative positions of components described in the following embodiments are arbitrarily determined and can be changed according to the configuration of a device to which the present invention is applied and a variety of other conditions. Further, except where particularly described, the scope of the present invention is not limited to the embodiments that will be specifically described below. 
     Unless otherwise particularly stated, the term “contrast medium” includes not only the contrast medium alone but the contrast medium and a chemical liquid containing a solvent and an additive different from the contrast medium. In the following description, unless otherwise particularly stated, the term “pixel value” includes a CT value, the sum or average of CT values of pixels contained in a region of interest (ROI), or an SD value (standard deviation value) in the region of interest in an imaged site having been contrasted. Further, the pixel value includes a value obtained by subtracting a value in an imaged site that has not been contrasted (CT value in imaged site in simple CT, for example) from any of the values described above. The region of interest can be set in advance, or a user can select a region of interest. 
     First Embodiment 
     A simulator (perfusion simulator)  20 , which predicts a time-dependent change in the pixel value in a tissue of a subject, includes a subject information acquisition section  11 , which acquires subject information on an examinee who is the subject, as shown in  FIG. 1 . The subject information includes, for example, a hemoglobin level, a body mass, a height, a body surface area, a cardiac function, a heart rate, a stroke volume, a cardiac output, an estimated glomerular filtration rate (eGFR), a creatinine level, an age, a gender, a fat-free body mass, a body mass index, a circulating blood volume, an examinee number (examinee ID), a history of diseases and side-effects of the examinee, the name of the examinee, the date of birth, a blood volume, and a blood flow speed. 
     The subject information acquisition section  11  acquires subject information inputted by a user via an input section  27  of the simulator  20 . The subject information acquisition section  11  may acquire the subject information from a memory unit  24  of the simulator  20  or an external storage device (server). The server is, for example, a radiology information system (RIS), a picture archiving and communication system (PACS), a hospital information system (HIS), an image examination system, and an image creation workstation. Further, the subject information acquisition section  11  may acquire the subject information from an imaging device  3  or an injection device  2  shown in  FIG. 5 . 
     The simulator  20  includes a protocol acquisition section  12 , which acquires a contrast medium injection protocol. The protocol acquisition section  12  acquires an injection protocol inputted by the user via the input section  27 . The injection protocol includes a chemical liquid injection period and a chemical liquid injection speed by way of example. The injection protocol may further include an injection method, a contrast medium injection location, an amount of injection, an injection timing, a contrast medium concentration, injection pressure, and acceleration of the injection speed. The protocol acquisition section  12  may acquire the injection protocol from the memory unit  24 , the external storage device, or the injection device  2 . 
     The injection protocol may include a contrast medium injection period and injection speed, a physiological saline injection period and injection speed, whether or not the contrast medium is injected by rear pushing, an increase or decrease in the injection speed, whether or not cross injection is performed, whether or not a speed linkage setting is made, the volume of an injection tube, and other pieces of information. The cross injection is an injection method for first injecting the contrast medium at a speed higher than the physiological saline injection speed until a set period elapses after the injection starts and then injecting not only the contrast medium in such a way that the injection speed gradually decreases but physiological saline in such a way that the injection speed gradually increases. The speed linkage setting is a setting that links the contrast medium injection speed and the physiological saline injection speed are so linked to each other that the two injection speeds are equal to each other. 
     The simulator  20  includes a tissue information acquisition section  13 , which acquires tissue information of the subject. The tissue information includes, for example, the number of compartments in the tissue (number of divided compartments of blood vessel and organ), the volume of the tissue (volume of vascular cavity), the volume of capillaries, the volume of an extracellular sap cavity, the amount of the blood flow per unit tissue (blood flow speed), a contrast medium permeating-out speed in the tissue (capillary permeable surface area), a contrast medium permeating-back speed in the tissue (capillary permeable surface area), and a pixel value specific to the tissue. The number of compartments may be so set as to be greater in a tissue having a large volume than in a tissue having a small volume. 
     The tissue information acquisition section  13  acquires tissue information inputted by the user via the input section  27 . The tissue includes the heart (right and left ventricles), blood vessels, kidney, ureter, and other organs and muscle. A prediction section  16 , when it acquires a pixel value specific to each tissue, predicts the degree of enhancement achieved by the contrast medium based on the pixel value specific to the tissue. The tissue information acquisition section  13  may acquire the tissue information from the memory unit  24 , the external storage device, or the injection device  2 . 
     The simulator  20  includes a chemical liquid information acquisition section  14 , which acquires the amount of contrast medium. The chemical liquid information acquisition section  14  further acquires chemical liquid information on a chemical liquid. The chemical liquid information acquisition section  14  acquires chemical liquid information inputted by the user via the input section  27 . The chemical liquid information includes, for example, a viscosity, an osmotic pressure ratio, an amount of contrast medium, an amount of physiological saline, a product ID, a product name, a chemical liquid classification, contained components, a concentration, an expiration date, a syringe volume, a syringe withstand pressure, a cylinder inner diameter, a piston stroke, and a lot number. 
     The chemical liquid information acquisition section  14  may acquire the chemical liquid information from the memory unit  24 , the external storage device, or the injection device  2 . The chemical liquid information acquisition section  14  may further acquire the chemical liquid information from a reader built in the injection device  2 . The reader reads chemical liquid information from a data carrier attached to a syringe incorporated in an injection head. The data carrier is, for example, an RFID chip, an IC tag, or a barcode. 
     The simulator  20  includes a target value acquisition section  15 , which acquires a target duration for which a target pixel value is maintained. The user can input at least one of the target pixel value and the target duration via the input section  27 . 
     The simulator  20  includes the prediction section  16 , which simulates a time-dependent change in the pixel value in a tissue of a subject to determine a predicted duration. The prediction section  16  simulates a time-dependent change in the pixel value in the tissue of the subject based on the acquired injection protocol and the amount of the contrast medium and determines the predicted duration for which a target pixel value is maintained. 
     Specifically, the prediction section  16  receives as the subject information the hemoglobin level (g/dL), the body mass (kg), the height (cm), the cardiac function (%), the heart rate (bpm), the body surface area (m 2 ), the cardiac output (L/min), and the eGFR from the subject information acquisition section  11 . The prediction section  16  may instead calculate at least one of the body surface area, the cardiac output, and the estimated glomerular filtration rate. For example, the body surface area can be calculated based on the body mass and the height by using the Fujimoto method, the DuBois method, or the Shinya method. The cardiac output can be calculated based on the body surface area, the cardiac function, and the heart rate. The eGFR can be calculated based on the creatinine level, the age, and the gender. 
     The cardiac function is so set by the user, provided that an average cardiac function is expressed by 100%, as to increase (120%, for example) when the cardiac function is superior to the average cardiac function and decrease (80%, for example) when the cardiac function is inferior to the average cardiac function. As a parameter that replaces the cardiac function, a measured cardiac output (L/min) or the ratio of a measured cardiac output to an average cardiac output may be used. 
     The prediction section  16  acquires, as the injection protocol, the contrast medium injection period (sec) and injection speed (mL/sec), the physiological saline injection period (sec) and injection speed (mL/sec), whether or not the cross injection is performed, and whether or not the speed linkage setting is made from the protocol acquisition section  12 . Further, the prediction section  16  acquires, as the tissue information, the number of compartments in the tissue, the volume of the tissue, the volume of the capillaries, the volume of the extracellular sap cavity, the blood flow speed, two types capillary permeable surface areas, and the pixel value specific to the tissue from the tissue information acquisition section  13 . The prediction section  16  acquires, as the chemical liquid information, the contrast medium concentration (mgI/mL), the osmotic pressure ratio, the viscosity (mPs.s), the amount of contrast medium (mL), and the total amount of iodine (mgI) from the chemical liquid information acquisition section  14 . The prediction section  16  then calculates the amount of the iodine per body mass (kg) (mgI/kg) from the total amount of the iodine and the body mass of the subject. 
     The prediction section  16  can acquire examination information, such as tube voltage (kV), via the input section  27 . The examination information may include an examination number (examination ID), an examination site, the date and time of the examination, the type of a chemical liquid, the name of the chemical liquid, and site information on the site to be imaged. The site information is information that allows identification of the site (range) selected as a target to be imaged. For example, the site information includes the name of an imaged site, the name of an imaging method, and the distance from a chemical liquid injection site to the imaged site. The prediction section  16  can further acquire additional information inputted by the user via the input section  27 , such as an analysis period (sec). The analysis period is the length of the period for which the prediction is performed and corresponds to the X-axis ( FIG. 2 ) length of a time-concentration curve (TDC curve). 
     The prediction section  16  then simulates a time-dependent change in the pixel value in the tissue of the subject in each of a plurality of compartment obtained by dividing the tissue of the subject along the blood flow direction. The simulation is performed based on the subject information, the injection protocol, and the tissue information. The prediction section  16  then associates the pixel values in each of the compartments over time with the respective tissues and stores the pixel values in the memory unit  24 . 
     The simulator  20  includes a control section  25 , which is formed, for example, of a CPU, and the memory unit  24 , which stores a control program. The control section  25  controls each portion of the simulator  20  according to the control program stored in the memory unit  24 . The control section  25  includes the subject information acquisition section  11 , the protocol acquisition section  12 , the tissue information acquisition section  13 , the chemical liquid information acquisition section  14 , and the target value acquisition section  15 . The sections described above are logically achieved as a variety of functions when the control section  25  carries out a variety of processes in correspondence with the control program implemented in the memory unit  24 . The control section  25  further functions as a display control section that controls a display section  26 . 
     The memory unit  24  includes a RAM (random access memory) that is a system work memory for allowing the control section  25  to operate, a ROM (read only memory) that stores a program or system software, or a hard disk drive. The memory unit  24  stores a simulation program that causes a computer (control section  25 ) to predict a time-dependent change in the pixel value in the tissue of the subject. 
     The simulation program causes the computer to function as the chemical liquid information acquisition section  14 , which acquires the amount of the contrast medium, the target value acquisition section  15 , which acquires a target duration for which a target pixel value is maintained, the protocol acquisition section  12 , which acquires the contrast medium injection protocol, and the prediction section  16 , which simulates the time-dependent change based on the injection protocol and the amount of the contrast medium and determines a predicted duration. The simulation program further causes the computer to function as the prediction section  16  that compares the predicted duration with the target duration and, when the predicted duration is shorter than the target duration, re-simulates the time-dependent change in a case where a greater amount of the contrast medium is injected than the amount of the contrast medium used in the simulation to redetermine the predicted duration, whereas, when the predicted duration is longer than the target duration, re-simulating the time-dependent change in a case where a smaller amount of the contrast medium is injected than the amount of the contrast medium used in the simulation to redetermine the predicted duration. The simulation program can be stored on a computer readable recording medium. 
     The control section  25  can instead control the variety of processes according to the control program and the simulation program stored on a CD (compact disc), a DVD (digital versatile disc), a CF (compact flash) card, or any other portable recording medium, or a server on the Internet or any other external recording medium. 
     The simulator  20  includes the display section  26 , which displays each of the compartments of the tissue in a color having a density according to the pixel value. The control section  25  then changes the grayscale of the compartment of the tissue displayed on the display section  26  according to a time-dependent change in the pixel value. To this end, the control section  25  reads the pixel value in the compartment at selected time from the memory unit  24  and changes the grayscale of the compartment. The display section  26  displays a main screen ( FIG. 2 ), an automatic optimization screen ( FIG. 3 ), and other operation screens. The display section  26  may display the injection protocol, the input state of the device, the setting state, and the result of the injection. 
     The simulator  20  includes the input section  27 , which is connected to the subject information acquisition section  11 , the protocol acquisition section  12 , the tissue information acquisition section  13 , and the chemical liquid information acquisition section  14 . The input section  27  can, for example, be a keyboard. A touch panel that serves both as the input section  27  and the display section  26  may instead be used. 
     [Prediction of Time-Dependent Change in Pixel Value] 
     Each tissue of the subject is divided into a plurality of compartments along the blood flow direction according to the number of compartments to which the tissue is divided acquired from the tissue information acquisition section  13 . The prediction section  16  predicts a time-dependent change in the pixel value in each of the compartments by dividing the volume of the tissue including the compartment under prediction, the volume of the capillaries in the tissue, and the volume of the extracellular sap cavity in the tissue by the number of divided compartments. For example, in a case where the number of divided compartments is 15, the prediction section  16  predicts a time-dependent change in the pixel value based on the quotients of the volume of the tissue, the volume of the capillaries, and the volume of the extracellular sap cavity divided by 15. 
     Tissues of a subject include the right ventricle, main artery, vein, artery, brain (head), upper limb, heart muscle (heart muscle in which right coronary artery is dominant, heart muscle in which anterior descending branch is dominant, heart muscle in which circumflex branch is dominant), lung, liver, stomach, spleen, pancreas, intestinal tract, kidney, ureter, lower limb, left ventricle, ascending main artery, descending main artery, and abdominal main artery. The contrast medium injected via the upper limb vein moves through the right ventricle, the lung, the left ventricle, and the main artery (ascending main artery, descending main artery) to each organ and then reaches the right ventricle via the vein. The contrast medium injected into the body is then discharged out of the body via the kidney and the ureter. 
     The prediction section  16  predicts a time-dependent change in the pixel value of each tissue sequentially from the right ventricle toward the upstream and downstream in the blood flow direction. For example, the prediction section  16  first performs the prediction on the right ventricle and then performs the prediction on a second tissue group including the main vein and the vein located on the upstream side of the right ventricle in the blood flow direction and the artery located on the downstream side of the right ventricle in the blood flow direction. That is, the prediction section  16  predicts a time-dependent change in the pixel value of each tissue sequentially from a tissue close to the contrast medium injection location toward tissues on the upstream and downstream in the blood flow direction. 
     The prediction section  16  uses a differential equation, for example, the following Equation 1 to determine a change in the pixel value in each tissue (blood vessels and organs) in the form of a time function. In Equation 1, C 1  represents the concentration of the contrast medium flowing into the compartment, C 2  represents the concentration of the contrast medium flowing out of the compartment, V represents the volume of the compartment, and Q represents the amount of blood flow per unit tissue (blood flow speed) in the compartment. 
     
       
         
           
             
               
                 
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     Further, to determine a change in the pixel value in a tissue other than the right ventricle, the left ventricle, and the blood vessels, the prediction section  16  takes the following speeds into account: the permeation-out speed of the contrast medium that passes from the capillaries to the extracellular sap cavity; and the permeation-back speed of the contrast medium that passes from the extracellular sap cavity to the capillaries. The prediction section  16  therefore uses differential equations, for example, the following Equations 2 and 3. In Equations 2 and 3, Vec represents the volume of the extracellular sap cavity, Cec represents the concentration of the contrast medium in the extracellular sap cavity, Viv represents the volume of the capillaries, Civ represents the concentration of the contrast medium in the capillaries, PS 1  represents the permeation-out speed, and PS 2  represents the permeation-back speed. 
     
       
         
           
             
               
                 
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     Solving the differential equations described above allows the elapsed period after the injection starts and a change in the pixel value (contrast medium concentration) in the form of a time function. The parameters used for the prediction in the case of stomach are, for example, as follows: the tissue volume greater than or equal to 120 mL but smaller than or equal to 160 mL; the capillary volume greater than or equal to 2 mL but smaller than or equal to 5 mL; the extracellular sap cavity volume greater than or equal to 12 mL but smaller than or equal to 18 mL; the amount of the blood flow per unit tissue (artery blood flow speed) greater than or equal to 120 mL/min but smaller than or equal to 180 mL/min; the permeation-out speed higher than or equal to 15 but lower than or equal to 25; and the permeation-back speed higher than or equal to 15 but lower than or equal to 25. 
     In the case of spleen, the following parameters are used: the tissue volume greater than or equal to 120 mL but smaller than or equal to 160 mL; the capillary volume greater than or equal to 10 mL but smaller than or equal to 15 mL; the extracellular sap cavity volume greater than or equal to 45 mL but smaller than or equal to 65 mL; the amount of the blood flow per unit tissue greater than or equal to 150 mL/min but smaller than or equal to 250 mL/min; the permeation-out speed higher than or equal to 15 but lower than or equal to 25; and the permeation-back speed higher than or equal to 15 but lower than or equal to 25. 
     In the case of pancreas, the following parameters are used: the tissue volume greater than or equal to 120 mL but smaller than or equal to 150 mL; the capillary volume greater than or equal to 3 mL but smaller than or equal to 6 mL; the extracellular sap cavity volume greater than or equal to 30 mL but smaller than or equal to 50 mL; the amount of the blood flow per unit tissue greater than or equal to 120 mL/min but smaller than or equal to 180 mL/min; the permeation-out speed higher than or equal to 15 but lower than or equal to 25; and the permeation-back speed higher than or equal to 15 but lower than or equal to 25. 
     In the case of intestinal tract, the following parameters are used: the tissue volume greater than or equal to 1800 mL but smaller than or equal to 20000 mL; the capillary volume greater than or equal to 30 mL but smaller than or equal to 40 mL; the extracellular sap cavity volume greater than or equal to 500 mL but smaller than or equal to 600 mL; the amount of the blood flow per unit tissue greater than or equal to 0.4 mL/min but smaller than or equal to 0.5 mL/min; the permeation-out speed higher than or equal to 150 but lower than or equal to 250; and the permeation-back speed higher than or equal to 150 but lower than or equal to 250. The permeation-out speed and the permeation-back speed can each be calculated in the form of the product of the capillary area and the permeability. For example, assuming that the total area of the capillaries in a human body is 800 m 2 , and a capillary area according to the mass of an organ is allocated to the organ. Assuming then that the permeability of each organ is 1 ml/min/g, and the permeation-out speed and the permeation-back speed can be calculated. 
     Further, the prediction section  16  takes into account the fact that the contrast medium diffuses between adjacent compartments. That is, the prediction section  16  predicts time-dependent changes in the pixel values in adjacent compartments in such a way that the concentration of the contrast medium in a high-concentration compartment is decreased but the concentration of the contrast medium in a low-concentration compartment is increased. In a case where there is a large difference in the concentration between the adjacent compartments, the prediction section  16  increases the amounts of increase and decrease in the contrast medium concentration. 
     In a case where the contrast medium osmotic pressure ratio is large, the prediction section  16  increases the amounts of increase and decrease in the contrast medium concentration. Further, in a case where the area where compartments are in contact each other is large, the prediction section  16  increases the amounts of increase and decrease in the contrast medium concentration. Specifically, in a case where different-tissue compartments are adjacent to each other, the prediction section  16  decreases the amounts of increase and decrease in the contrast medium concentration because the contact area decreases. In a case where same-tissue compartments are adjacent to each other, the prediction section  16  increases the amounts of increase and decrease in the contrast medium concentration because the contact area increases. 
     [Discharge of Contrast Medium] 
     Part of the contrast medium injected into an actual body does not recirculate because it is discharged out of the body via the kidney and the ureter. The prediction section  16  therefore calculates the discharged amount of the contrast medium based on a predetermined discharge speed and subtracts the discharged amount from the amount of the contrast medium in the capillaries in the kidney for the simulation. As a result, part of the contrast medium having reached the kidney is subtracted from the total amount of contrast medium in the overall body (blood plasma). Specifically, the prediction section  16  allocates the contrast medium having reached the kidney to the capillaries, the extracellular sap cavity, and the cell parenchym. The prediction section  16  then divides the contrast medium allocated to the capillaries in the kidney into three parts and allocates the three parts to the kidney artery, the extracellular sap cavity in the kidney, and the ureter. That is, the prediction section  16  moves the discharged amount of the contrast medium from the kidney capillaries to the ureter for the simulation. The contrast medium moved to the ureter does not return into the body. As a result, the contrast medium moved to the ureter is subtracted from the total amount of contrast medium in the overall body. 
     The contrast medium moved to the ureter increases in proportion to the concentration of the contrast medium in the kidney capillaries. That is, the contrast medium discharge speed (mL/sec) increases in proportion to the concentration of the contrast medium in the capillaries. The contrast medium concentration is the ratio of the contrast medium in the tissue (compartment) to the contrast medium in the blood on a unit time (10 msec, for example) basis. The contrast medium injected into an actual body increases in proportion to eGFR. The prediction section  16  then multiplies eGFR by an adjustment coefficient and further multiplies the contrast medium concentration by the value that is the result of the first multiplication to determine the contrast medium discharge speed. The adjustment coefficient is greater than zero but smaller than five. The total amount of the contrast medium in the overall body decreases over time as a result of the simulation of the discharge of the contrast medium, whereby the simulation can be performed with higher precision. Further, the prediction section  16  may subtract the contrast medium in the ureter after a predetermined period elapses so that the simulation reflects the fact that the contrast medium in the ureter is pushed by urine and moved into the bladder. 
     After the simulation is completed, the prediction section  16  causes the memory unit  24  to successively store the results of the simulation. The results of the simulation include information on the pixel value for each point of time related to a tissue. The display section  26  then diagrammatically displays a predicted image of each tissue including a plurality of compartments. Further, the control section  25  reads the pixel values from the memory unit  24  and controls the display section  26  in such a way that the grayscale of each compartment is changed according to the time-dependent change in the pixel value. 
     [Main Screen] 
     A main screen as an example of a screen displayed on the display section  26  will be described with reference to  FIG. 2 . The main screen is an operation screen that allows the user to input a variety of numerical values, and a predicted image  41  is disposed on the right of the main screen. A condition setting field  42  is disposed in an upper left portion of the main screen, and a time-concentration curve field  43  is disposed in a central upper portion of the main screen. Display buttons  44  are disposed in a central lower portion of the main screen, and a patient setting field  45  is disposed in a lower right portion of the main screen. 
     In the time-concentration curve field  43 , the horizontal axis corresponds to the elapsed time (sec) after the injection starts, and the vertical axis corresponds to the pixel value (HU). It is, however, noted that in a case where the total amount of the contrast medium is displayed, the vertical axis corresponds to the amount of the contrast medium (mL). In the time-concentration curve field  43 , time-concentration curves associated with a plurality of tissues can be so displayed as to be superimposed on one another. In this case, the control section  25  displays the time-concentration curves in different colors. 
     The user can operate a scrollbar  431  below the time-concentration curve field  43  to move a current time point bar  432  rightward and leftward in  FIG. 2 . The control section  25  reads from the memory unit  24  the pixel value in each compartment at the time selected by the user&#39;s operation of the scrollbar  431 . The control section  25  then reflects the read pixel values in the predicted image  41  and causes the display section  26  to display the image. For example, in  FIG. 2 , the point of time when 31.0 seconds have elapsed since the injection started has been selected, and a predicted image  41  at the selected point of time is displayed. In an initial setting, a predicted image  41  at the injection start point of time, that is, at the point of time when 0 seconds have elapsed since the injection started is displayed. 
     The user can operate a scrollbar  433  on the left of the time-concentration curve field  43  to move a pixel value bar  434  upward and downward in  FIG. 2 . The control section  25  displays, in a position below the scrollbar  433 , the pixel value in the position selected by the user&#39;s operation of the scrollbar  433 . For example, in  FIG. 2 , a pixel value 350 HU has been selected and displayed in the position below the scrollbar  433 . Further, the user can select a Y-axis fixing check box  435  to fix the maximum on the Y axis (in  FIG. 2 , Y-axis fixing setting has not been selected). For example, in a case where the maximum on the Y axis is fixed at  400 , the maximum on the Y axis is maintained at  400  even when only the total amount of the contrast medium (50.0 mL at the maximum, for example) is displayed. On the other hand, in the case where the Y-axis fixing setting has not been selected, and when only the total amount of the contrast medium of, for example, 50.0 mL at the maximum is displayed, the maximum on the Y axis is changed to 50. 
     The user can manipulate the image by operating operation buttons  411  below the predicted image  41 . The operation buttons  411  include a stop button, a reproduction button, a triple-speed reproduction button, a 10-times-speed reproduction button, a 30-times-speed reproduction button, and a reset button sequentially from the left of the main screen. When the user selects the reproduction button, the predicted image  41  is continuously reproduced in the form of motion images over the elapsed time. When the user selects any of the triple-speed reproduction button, the 10-times-speed reproduction button, and the 30-times-speed reproduction button, the motion image reproduction speed increases. The user can therefore visually recognize the position of the contrast medium in each tissue at a desire point of time. The current time point bar  432  moves along the X axis in correspondence with the elapsed time when the predicted image  41  is continuously reproduced. When the user selects the stop button, the reproduction temporarily is suspended. When the user selects the reset button, the reproduction is terminated, and the predicted image  41  returns to the initial setting (point of time when zero seconds have elapsed after injection started). 
     To select a window width (WW) and a window level (WL), a plurality of WL/WW selection buttons  412  are disposed on the right of the predicted image  41  in the main screen. The window width corresponds to the range of the contrast of the pixel value, and the window level corresponds to the brightness of the screen. In  FIG. 2 , a window width of 400 and a window level of 50 have been set. In a case where a pixel value is smaller than the value obtained by subtracting half the value of the window width from the value of the window level, the display section  26  displays the pixel value in black. In a case where a pixel value is greater than the value obtained by adding half the value of the window width to the value of the window level, the display section  26  displays the pixel value in white. Further, the display section  26  displays a non-contrasted tissue by using the specific pixel value. Input fields into which the window width and the window level are inputted may instead be disposed in the main screen. 
     A plurality of display buttons  44 , which allow the user to select a tissue to be displayed, are disposed in a portion below the time-concentration curve field  43 . The user can select a tissue to be displayed in the time-concentration curve field  43  from the display buttons  44 . In  FIG. 2 , a liver artery button  442  and a total contrast medium amount button  441  have been selected. When the user selects the total contrast medium amount button  441 , a change in the total amount of the contrast medium left in the body is displayed in the time-concentration curve field  43 . 
     In the condition setting field  42 , the chemical liquid information, the injection protocol, the tube voltage, and the analysis period can be set. Specifically, the user can operate a chemical liquid pulldown menu  421  to select one of a plurality of chemical liquid names. When the user selects a chemical liquid, the chemical liquid information acquisition section  14  acquires the contrast medium concentration, the viscosity, the osmotic pressure ratio, and the amount of the contrast medium corresponding to the chemical liquid name selected by the user. The contrast medium concentration, the viscosity, the osmotic pressure ratio, and the amount of the contrast medium have been inputted in advance to the memory unit  24 . 
     The chemical liquid information acquisition section  14  can instead acquire the contrast medium concentration, the viscosity, the osmotic pressure ratio, and the amount of the contrast medium inputted by the user via the input section  27 . Specifically, when the user selects the contrast medium concentration button, the control section  25  displays a contrast medium concentration input screen on the display section  26 . The user can then input a desired contrast medium concentration via the input screen. Similarly, the user can input the contrast medium concentration, the viscosity, the osmotic pressure ratio, and the amount of the contrast medium. According to the input, the control section  25  displays the contrast medium concentration, the viscosity, the osmotic pressure ratio, and the amount of the contrast medium acquired by the chemical liquid information acquisition section  14  in a contrast medium display field. 
     In a contrast medium setting field, the user can select an injection period button  422  to input a contrast medium injection period in the injection protocol. When the user selects the injection period button  422 , the control section  25  displays an injection period input screen on the display section  26 . The user can then input a desired injection period via the input screen. Similarly, the user can select an injection speed button  423  to input a contrast medium injection speed in the injection protocol. Further, a per-body-mass contrast medium amount button may be disposed in the contrast medium setting field so that a contrast medium amount per body mass (mgI/kg) can be inputted. In this case, when the contrast medium amount per body mass is inputted, the prediction section  16  multiplies the amount of the contrast medium per body mass by the body mass to automatically change the amount of the contrast medium. 
     The user can select a cross injection check box  425  to select whether or not the cross injection is performed. In  FIG. 2 , the selection has been so made that the cross injection is performed. Similarly, the user can select a speed linkage setting check box  426  to select whether or not the speed linkage setting is made. In  FIG. 2 , the speed linkage setting has not been selected. 
     In a physiological saline setting field, the user can select an injection period button  422  to input a physiological saline injection period in the injection protocol. When the user selects the injection period button  422 , the control section  25  displays an injection period input screen on the display section  26 . The user can then input a desired injection period via the input screen. Similarly, the user can select an injection speed button  423  and a physiological saline amount button  424  to input a physiological saline injection speed and a physiological saline amount in the injection protocol, respectively. 
     An injection amount display screen  427  is disposed in a portion below the physiological saline setting field. In the injection amount display screen  427 , the horizontal axis represents the elapsed time after the injection starts, and the vertical axis represents the injection speed. In the injection amount display screen  427 , the control section  25  displays the injection amount in the injection protocol acquired by the protocol acquisition section  12 . In  FIG. 2 , the setting has been so made that the cross injection is performed, and the injection amount display screen  427  displays a graph showing that the contrast medium is injected at an injection speed of 4.0 mL/sec and the injection speed gradually decreases after a predetermined period elapses. The graph is so drawn that the area of the region surrounded by the solid line represents the amount of the injected contrast medium. In this case, the prediction section  16  predicts a time-dependent change in the pixel value in each tissue with the cross injection reflected. 
     Further, in the injection amount display screen  427 , a graph representing the injection of the physiological saline is so displayed as to show that after the contrast medium injection starts and a predetermined period then elapses, injection of physiological saline starts, and the injection speed of the physiological saline gradually increases and reaches the injection speed of 4.0 mL/sec after a predetermined period elapses. The graph is so drawn that the area of the region surrounded by the dotted line represents the amount of the injected physiological saline. The region representing the amount of the injected contrast medium and the region representing the amount of the injected physiological saline may be so displayed as to be filled with different colors. 
     In a case where the selection has been so made that no cross injection is performed, the graph is so displayed as to show that the injection of the physiological saline starts after the injection of the contrast medium is completed. For example, the graph is so displayed as to show that the contrast medium is injected at the injection speed of 4.0 mL/sec after the injection starts, the physiological saline is injected at the injection speed of 4.0 mL/sec when the injection of the contrast medium of 50.0 mL is completed, and the injection is terminated when the injection of the physiological saline of 25.0 mL is completed. 
     In a case where the speed linkage setting has been selected, the contrast medium injection speed and the physiological saline injection speed are so set as to be equal to each other. For example, in a case where the contrast medium injection speed is changed from 4.0 mL/sec to 5.0 mL/sec, the physiological saline injection speed is automatically set at 5.0 mL/sec. In this case, input of a physiological saline injection speed may be prohibited. Further, in a case where the physiological saline injection speed is changed, the contrast medium injection speed may be automatically set. 
     A tube voltage button  428 , an analysis period button  429 , and an update button  420  are disposed in a portion below the injection amount display screen  427 . The user can select the tube voltage button  428  to set the tube voltage. When the user selects the tube voltage button  428 , the control section  25  displays a tube voltage input screen on the display section  26 . The user can then input desired tube voltage via the input screen. Similarly, the user can select the analysis period button  429  to input an analysis period. 
     The patient setting field  45  is disposed in a portion below the operation buttons  411 . In the patient setting field  45 , the body mass, height, cardiac function, and heart rate can be set. The control section  25  displays in advance the body mass, height, cardiac function, and heart rate in the patient setting field  45  based on the subject information acquired by the subject information acquisition section  11 . The subject information acquisition section  11  can also acquire a body mass, height, cardiac function, and heart rate inputted by the user via the input section  27 . Specifically, when the user selects a body mass button  451 , the control section  25  displays a body mass input screen on the display section  26 . The user can then input a body mass of the subject via the input screen. Similarly, the user can select a height button  452 , a cardiac function button  453 , and a heart rate button  454  to input a height, cardiac function, and heart rate of the subject, respectively. 
     A body surface area field and a cardiac output field are disposed in the patient setting field  45 . The prediction section  16  calculates the body surface area based on the body mass and height of the subject acquired by the subject information acquisition section  11 . The control section  25  displays the calculated body surface area in the body surface area field. When the user inputs a body mass or any other parameter of the subject, the subject information acquisition section  11  acquires the inputted body mass or any other parameter. Similarly, the prediction section  16  calculates the cardiac output based on the body surface area, the cardiac function, and the heart rate of the subject acquired by the subject information acquisition section  11 . The control section  25  displays the calculated cardiac output in the cardiac output field. 
     An eGFR field, a creatinine level button, an age button, and a gender button may further be disposed in the patient setting field  45 . In this case, the user can select the creatinine level button, the age button, and the gender button to input a creatinine level, age, and gender of the subject, respectively. The subject information acquisition section  11  acquires the inputted creatinine level and the like. The prediction section  16  calculates eGFR based on the acquired creatinine level, age, and gender of the subject. The control section  25  displays the calculated eGFR in the eGFR field. Instead, the subject information acquisition section  11  can also acquire the heart rate from an external measurement tool or the memory unit  24 . Further, the subject information acquisition section  11  can also acquire the stroke volume or the cardiac output from an external measurement tool. In the case where the stroke volume is acquired, the prediction section  16  multiplies the stroke volume by the heart rate to calculate the cardiac output. 
     Upon completion of the setting, the user selects the update button  420 . A variety of pieces of inputted information are thus acquired. The prediction section  16  then performs the simulation according to the variety of pieces of acquired information and causes the memory unit  24  to store the result of the simulation. The control section  25  then reads the result of the simulation from the memory unit  24  and displays a predicted image  41  of a tissue corresponding to the display button  44  selected by the user. Similarly, the control section  25  displays a time-concentration curve associated with the tissue corresponding to the display button  44  selected by the user in the time-concentration curve field  43 . 
     An automatic optimization tab  46  and a tissue setting tab  47  are disposed on the right of a main screen tub in  FIG. 2 . When the user selects the automatic optimization tab  46 , the control section  25  displays an automatic optimization screen ( FIG. 3 ) on the display section  26 . The user can then automatically optimize the injection protocol via the automatic optimization screen. When the user selects the tissue setting tab  47 , the control section  25  displays a tissue setting screen (not shown) on the display section  26 . The user can then input tissue information (for example, volume of tissue, volume of capillaries, volume of extracellular sap cavity, an amount of blood flow per unit tissue, permeation-out speed of contrast medium in tissue, and permeation-back speed of contrast medium in tissue) via the tissue setting screen. 
     [Automatic Optimization Screen] 
       FIG. 3  shows the automatic optimization screen for automatic optimization of the injection protocol. An optimization setting field is disposed in an upper portion of the automatic optimization screen, and preset buttons  461  and load buttons  462  are disposed in a lower portion of the automatic optimization screen. When the user selects an optimization button  467  in the automatic optimization screen, the prediction section  16  automatically optimizes the injection protocol. 
     A target pulldown menu  463 , a target pixel value button  464 , a target duration button  465 , a maximum contrast medium amount button  466 , an optimization button  467 , a time fixing check box  468 , and a speed fixing check box  469  are disposed in the optimization setting field. The user can operate the target pulldown menu  463  to select one of a plurality of tissues. When the user selects a target tissue, the tissue information acquisition section  13  acquires tissue information corresponding to the target tissue selected by the user. 
     The user can select the target pixel value button  464  to input a target pixel value. When the user selects the target pixel value button  464 , the control section  25  displays a target pixel value input screen on the display section  26 . The user can then input a desired target pixel value via the input screen. Similarly, the user can select the target duration button  465  and the maximum contrast medium amount button  466  to input the target duration and a maximum amount of contrast medium, respectively. 
     The target value acquisition section  15  acquires the inputted target pixel value and the target duration. The chemical liquid information acquisition section  14  acquires the inputted maximum contrast medium amount. The chemical liquid information acquisition section  14  may instead acquire the amount of the contrast medium filled in the syringe as the maximum contrast medium amount based on the chemical liquid name selected by the user. The chemical liquid information acquisition section  14  may restricts the maximum contrast medium amount in such a way that the ratio between the amount of single administered contrast medium (gI) and eGFR is smaller than one. The chemical liquid information acquisition section  14  may further refer to a table that relates eGFR and the maximum contrast medium amount to each other to acquire the maximum contrast medium amount. The table is stored in the memory unit  24  in advance. 
     The user can further select the time fixing check box  468  or the speed fixing check box  469  to select a time fixing or speed fixing condition. In  FIG. 3 , the speed fixing condition has been selected. When the speed fixing condition is selected, the prediction section  16  performs the re-simulation with no change in the injection speed. When the time fixing condition is selected, the prediction section  16  performs the re-simulation with no change in the injection period. 
     The user can select any of the preset buttons  461  to save the setting inputted into the optimization setting field at the time of the button selection as any of presets  1  to  4 . When any of the preset buttons  461  is selected, the prediction section  16  causes the memory unit  24  to store the setting inputted as any of the presets  1  to  4  according to the selected button. The user can select any of the load buttons  462  to read the setting saved as the corresponding one of the presets  1  to  4 . When any of the load buttons  462  is selected, the prediction section  16  reads any of the setting stored as the corresponding one of the presets  1  to  4  from the memory unit  24  according to the selected button. The prediction section  16  then reflects the read setting in the target pixel value and the like. 
     [Automatic Optimization] 
     The optimization will be described below with reference to the flowchart shown in  FIG. 4 . When the user selects the optimization button  467  (S 101 ), the prediction section  16  acquires a variety of pieces of information (S 102 ), as shown in  FIG. 4 . Specifically, the prediction section  16  acquires a contrast medium injection protocol from the protocol acquisition section  12 . The prediction section  16  further acquires a maximum contrast medium amount from the chemical liquid information acquisition section  14  and acquires a target pixel value and a target duration from the target value acquisition section  15 . The prediction section  16  then simulates a time-dependent change in the pixel value in a tissue of a subject based on the acquired maximum contrast medium amount in a case where half the maximum contrast medium amount is injected as the contrast medium amount to be used according to the acquired injection protocol. The prediction section  16  then determines a predicted duration from the result of the simulation (S 103 ). 
     The prediction section  16  then compares the determined predicted duration with the target duration (S 104 ). In a case where the predicted duration is shorter than the target duration (YES in S 105 ), the prediction section  16  re-simulates a time-dependent change in the pixel value in the tissue of the subject in a case where a greater amount of contrast medium than the amount of the contrast medium used in the preceding simulation is injected. That is, the prediction section  16  increases the amount of contrast medium to be used (S 106 ). Specifically, a contrast medium amount V n+1  to be used in the re-simulation is calculated by using the following Equation 4, where V n  represents the amount of the contrast medium used in the preceding simulation, T G  represents the target duration, and T A  represents the predicted duration obtained in the preceding simulation. A weighting coefficient W in Equation 4 is, for example, 0.5. The weighting coefficient W can be lowered according to the number of repetitions of the re-simulation.
 
[Math. 4]
 
 V   n+1   =V   n   +W ( T   G   −T   A )  (Equation 4)
 
     For example, assuming that the contrast medium amount V n  used in the preceding simulation is 50 mL, the target duration T G  is 8 seconds, the predicted duration T A  is 7.5 seconds, and the weighting coefficient W is 0.5, a contrast medium amount to be used in the re-simulation is calculated to be 50.25 mL, as shown by the following Equation 5:
 
[Math. 5]
 
50.25=50+0.5(8−7.5)  (Equation 5)
 
     The prediction section  16  changes at least one of the contrast medium injection speed and injection period to perform the re-simulation. That is, since the amount of the contrast medium to be used increases, the prediction section  16  changes at least one of the injection speed and the injection period in the injection protocol. Specifically, in the case where the speed fixing condition is selected, the prediction section  16  does not change the injection speed but prolongs the injection period in the injection protocol. The injection period therefore is prolonged, resulting in an increase in the amount of the contrast medium to be used. In the case where the time fixing condition is selected, the prediction section  16  does not change the injection period but increases the injection speed in the injection protocol. The injection speed per unit time therefore increases, resulting in an increase in the amount of the contrast medium to be used. 
     In a case where the predicted duration is longer than the target duration (YES in S 107 ), the prediction section  16  re-simulates a time-dependent change in the pixel value in the tissue of the subject in a case where a smaller amount of contrast medium than the amount of the contrast medium used in the preceding simulation is injected. That is, the prediction section  16  decreases the amount of the contrast medium to be used (S 108 ). For example, assuming that the amount of the contrast medium V n  used in the preceding simulation is 50 mL, the target duration T G  is 8 sec, the predicted duration T A  is 8.5 sec, and the weighting coefficient W is 0.5, an amount of the contrast medium to be used in the re-simulation is calculated to be 49.75 mL, as shown by the following Equation 6:
 
[Math. 6]
 
49.75=50+0.5(8−8.5)  (Equation 6)
 
     Similarly, the prediction section  16  changes at least one of the contrast medium injection speed and injection period to perform the re-simulation. That is, since the amount of the contrast medium to be used decreases, the prediction section  16  changes at least one of the injection speed and the injection period in the injection protocol. Specifically, in the case where the speed fixing condition is selected, the prediction section  16  does not change the injection speed but shortens the injection period in the injection protocol. The injection period therefore shortens, resulting in a decrease in the amount of the contrast medium to be used. In the case where the time fixing condition is selected, the prediction section  16  does not change the injection period but decreases the injection speed in the injection protocol. The injection speed per unit time therefore decreases, resulting in a decrease in the amount of the contrast medium to be used. 
     The prediction section  16  performs the re-simulation of a time-dependent change in the pixel value in the tissue of the subject in a case where the amount of the contrast medium to be used that has been calculated according to the changed injection protocol is injected (S 109 ). The prediction section  16  then redetermines a predicted duration from the result of the re-simulation. The prediction section  16  then causes the memory unit  24  to store the result of the re-simulation and the injection protocol used in the re-simulation. In a case where a termination condition is satisfied (YES in S 110 ), the re-simulation is terminated. The termination condition is a case where the predicted duration coincides with the target duration, a case where the re-simulation has been performed by a predetermined number (40 times, for example), a case where a predetermined period (10 seconds, for example) has elapsed since the re-simulation started, or a case where the amount of change is smaller than or equal to a predetermined threshold. The condition that the amount of change is smaller than or equal to a predetermined threshold is a condition that the difference between the predicted duration in the current re-simulation and the predicted duration in the preceding re-simulation is smaller than or equal to a predetermined threshold (0.01 seconds, for example). 
     In a case where the termination condition is not satisfied (NO in S 110 ), the prediction section  16  compares again the determined predicted duration with the target duration (S 104 ). In the case where the predicted duration is shorter than the target duration, the prediction section  16  re-simulates a time-dependent change in the pixel value in the tissue of the subject in a case where a larger amount of contrast medium is injected. In the case where the predicted duration is longer than the target duration, the prediction section  16  re-simulates a time-dependent change in the pixel value in the tissue of the subject in a case where a smaller amount of contrast medium is injected. The prediction section  16  then redetermines the predicted duration from the result of the re-simulation. 
     When the re-simulation is completed, the prediction section  16  causes the memory unit  24  to store as an optimum injection protocol an injection protocol corresponding to a simulation result showing the smallest difference between the target duration and the predicted duration out of the stored re-simulation results. The prediction section  16  may instead cause the memory unit  24  to store as the optimum injection protocol an injection protocol corresponding to a simulation result showing that the predicted duration is longer than or equal to the target duration and the amount of the contrast medium to be used is the smallest out of the stored re-simulation results. 
     The control section  25  subsequently closes the automatic optimization screen and opens the main screen. The prediction section  16  reflects at the same time the optimum injection protocol conditions (contrast medium amount, contrast medium injection period and injection speed, physiological saline amount, and physiological saline injection period and injection speed) in the contrast medium setting field. The control section  25  then displays the optimum injection protocol in place of the injection protocol before the optimization. The control section  25  further reads a time-concentration curve based on the simulation result corresponding to the optimum injection protocol and displays the read time-concentration curve in the time-concentration curve field  43 . Similarly, the control section  25  reads and displays a predicted image  41  and terminates the automatic optimization. 
     In a case where a result of the re-simulation shows that the target pixel value or the target duration has not been reached, the control section  25  displays a simulation condition correction proposal. That is, in a case where the target has not been achieved when the re-simulation is completed, the control section  25  displays a simulation condition correction proposal on the display section  26 . The correction proposal proposes to the user by way of example a decrease in the tube voltage, an increase in the contrast medium amount (increase in contrast medium amount by 50%, for example), an increase in the injection speed (increase in injection speed by 50%, for example), or extension of the analysis period. 
     The invention according to the first embodiment described above allows a simulation of a time-dependent change in the pixel value in a tissue of a subject in a case where an injection protocol that maintains a target pixel value over a target duration is used. Further, the simulator  20  according to the first embodiment can perform higher-precision prediction that approximates to a time-dependent change in the pixel value in an actual tissue. Further, an optimum injection protocol that maintains a target pixel value over a target duration can be obtained. 
     A helical scan box may further be disposed in the main screen. The user can select the helical scan box to input a bed movement speed (cm/sec). When the helical scan box is selected, the control section  25  acquires a delay period due to the helical scan. The delay period corresponds to an elapsed period from the time when the head is imaged to the time when each tissue is imaged (bed movement period) and is determined based on the length from the upper end of the predicted image  41  to each tissue. 
     The control section  25  then reads the pixel value at the time which is selected by the user (current point of time) plus the delay period from the memory unit  24 . That is, the control section  25  reads the pixel value in each tissue at the time obtained by adding the acquired delay period to the current point of time. A predicted image  41  for when the helical scan is performed can thus be produced. For example, in a case where the current point of time is immediately after the injection starts (0 seconds), the control section  25  shows the pixel value in the brain immediately after the injection starts and the pixel value in the right ventricle 5 seconds after the injection starts. Further, the control section  25  shows the pixel value in the liver 7.5 seconds after the injection starts. 
     Second Embodiment 
     An imaging system  100  including the simulator  20  ( FIG. 1 ) will be described with reference to  FIG. 5 , which is a schematic view of an injection device and an imaging system. In the second embodiment, the simulator  20  is incorporated in at least one of the imaging system  100  and the injection device  2 . The second embodiment will be described about points different from those in the first embodiment, and the components described in the first embodiment will not be described. Unless otherwise particularly described, a component having the same reference character has roughly the same action and function, and an advantageous effect provided by the component is roughly the same. 
     The imaging system  100  includes the injection device  2 , which injects a contrast medium, and a medical imaging device  3 , which is connected to the injection device  2  via a wire or wirelessly and captures an image of a subject, as shown in  FIG. 5 . The imaging device  3  is, for example, a magnetic resonance imaging (MRI) device, a computed tomography (CT) device, an angiographic imaging device, a positron emission tomography (PET) device, a single photon emission computed tomography (SPECT) device, a CT angiographic device, an MR angiographic device, an ultrasonic diagnosis device, and a blood vessel imaging device, or any of a variety of other medical imaging device. The following description will be made with reference to a CT device. 
     The imaging device  3  includes an imaging section  31 , which captures an image of the subject according to an imaging plan, and a control device  32 , which controls the entire imaging device  3 . The imaging plan includes, for example, a site to be imaged, an effective tube voltage, a model name, a manufacturer name, an imaging period, tube voltage, an imaging range, a rotational speed, a helical pitch, an exposure period, a dose, and an imaging method. The control device  32  controls the imaging section  31  according to the imaging plan to cause the imaging section  31  to capture an image of the subject. The control device  32  also functions as the simulator  20 . The control device  32  can communicate with the imaging section  31 , the injection device  2 , and a server (external storage device) via a wire or wirelessly. 
     The imaging section  31  includes a bed, an X-ray source that irradiates the subject with X-rays, and an X-ray detector that detects the X-rays having passed through the subject. The imaging section  31  captures a see-through image of the subject by exposing the subject to the X-rays and performing inverse projection of the interior of the subject based on the X-rays having passed through the subject. The imaging section  31  may instead perform the imaging by using a radio wave or an ultrasonic wave. 
     The imaging device  3  includes a display  33  as a display section. The display  33  is connected to the control device  32  and displays the input state and setting state of the imaging device  3 , the result of the imaging performed by the imaging device  3 , and a variety of other pieces of information. The control device  32  and the display  33  can instead be integrated with each other. Further, the imaging device  3  includes a user interface, such as a keyboard, as an input section  34 . The user can input chemical liquid information, an injection protocol, tissue information, subject information, and a target value via the input section  34  to the imaging device  3 . 
     The injection device  2  includes an injection head  21 , which injects a contrast medium according to the injection protocol. The injection device  2  injects a chemical liquid with which the syringe is filled, for example, physiological saline and a variety of contrast mediums into the subject. The injection device  2  further includes a stand  22 , which holds the injection head  21 , and a console  23 , which is connected to the injection head  21  via a wire or wirelessly. 
     The console  23  functions as a control device that controls the injection head  21  and also functions as the simulator  20 . The console  23  includes a touch panel  26 , which functions as an input display section and can communicate with the injection head  21  and the imaging device  3  via a wire or wirelessly. The touch panel  26  can display the injection protocol, the input state and setting state of the injection device, the result of the injection, and a variety of other pieces of information. The injection device  2  may include a display as the display section and a keyboard as the input section in place of the touch panel  26 . 
     The injection device  2  may include a control device connected to the injection head  21  and a display section (touch panel display, for example) that is connected to the control device and displays a chemical liquid injection situation in place of the console  23 . The control device also functions as the simulator  20 . The injection head  21  and the control device can be integrated with the stand  22 . Further, a ceiling hanging member can be provided in place of the stand  22 , and the injection head  21  can be hung from the ceiling via the ceiling hanging member. 
     The injection device  2  may include a remote operation device (hand switch or footswitch, for example) that remotely operates the injection head  21 . The remote operation device can remotely operate the injection head  21  to start or stop the injection. Further, the injection device  2  may include a power source or a battery. The power source or the battery can be provided in the injection head  21  or the control device or may be provided separately therefrom. 
     The injection head  21  includes a syringe holder on which the syringe filled with the chemical liquid is mounted and a drive mechanism that pushes the chemical liquid in the syringe according to the injection protocol. The injection head  21  includes an operation section  212 , via which the action of the drive mechanism is inputted. The operation section  212  is provided, for example, with a forward button that causes the drive mechanism to produce forward motion, a backward button that causes the drive mechanism to produce backward motion, and a final check button. The injection head  21  may further include a head display that displays the injection conditions, the injection status, the input and setting states of the injection device, and a variety of injection results. 
     To inject the contrast medium, an extension tube or any other attachment is connected to a front-end portion of the syringe incorporated in the injection head  21 . When the injection preparation is completed, the user presses the final check button on the operation section  212 . The injection head  21  then waits in a state in which it is ready for injection. When the injection starts, the contrast medium pushed out of the syringe is injected through the extension tube into the body of the subject. 
     The syringe incorporated in the injection head  21  may include a pre-filled syringe having a data carrier, such as an RFID chip, an IC tag, and a barcode, and a variety of other syringes. The injection head  21  includes a reader (not shown) that reads the data carrier attached to the syringe. The data carrier stores chemical liquid information on the chemical liquid. Further, the injection head  21  may include at least three syringe holders or only one syringe holder. 
     The injection device  2  can receive information from a server (external storage device) that is not shown and transmit information to the server. The imaging device  3  can also receive information from the server and transmit information to the server. The server is, for example, an RIS (radiology information system), a PACS (picture archiving and communication system), or an HIS (hospital information system). 
     The server stores an examination order in advance. The examination order includes subject information on the subject and examination information on the contents of the examination. The server can store information on the result of the imaging, such as image data transmitted from the imaging device  3 , and information on the result of the injection transmitted from the injection device  2 . To operate the injection device  2  and the imaging device  3 , an external image examination system or an image creation workstation can also be used. 
     In the case of the imaging device  3  according to the second embodiment, the user can operate the imaging device  3  while checking the predicted image  41  on the display  33 . The imaging device  3  can change the imaging plan according to the result of the prediction performed by the prediction section  16 . Specifically, the imaging device  3  can change, for example, the tube voltage or the tube current in such a way that a result of the simulation shows that a target pixel value or a target duration is reached. 
     Further, in the case of the injection device  2  according to the second embodiment, the user can operate the injection device  2  while checking the predicted image  41  on the console  23 . The injection device  2  can change, for example, the injection speed or the injection period in such a way that the injection protocol coincides with an optimum injection protocol obtained by the automatic optimization. 
     The present invention has been described above with reference to the embodiments, but the present invention is not limited to the embodiments described above. An invention changed to the extent that the invention does not depart from the present invention and an invention equivalent to the present invention also fall within the scope of the present invention. Further, the embodiments and variations can be combined with each other as appropriate to the extent that the combination does not depart from the present invention. 
     For example, the simulator  20  may be incorporated in an external computer connected to at least one of the imaging device  3  and the injection device  2  via a wire or wirelessly. In this case, the simulator  20  transmits the result of the simulation and an optimum injection protocol to the imaging device  3  and the injection device  2 , respectively. 
     The display section  26  can also display a coronary cross-sectional predicted image  41  as well as a horizontal cross-section of the body. Further, the display section  26  may place compartments in such a way that each tissue is displayed independently and display each of the compartments in a color having a density according to the pixel value. Further, the display section  26  may display each of the compartments in a color other than black and white. 
     The control section  25  may control the display section  26  in such a way that the number of compartments varies on a tissue basis. In this case, the control section  25  displays each tissue including the number of compartments set by the user or the number of compartments stored in the memory unit  24  in advance. Further, the control section  25  may display a target pixel value and a predicted duration in the time-concentration curve field  43 . 
     The prediction section  16  may take into account a change in the amount of blood flow per unit tissue (blood flow speed) due to injection of a chemical liquid. That is, in a case where the chemical liquid injection speed is faster than a typical blood flow speed, the prediction section  16  can subtract the blood flow speed from the injection speed to produce a difference, add the difference to the blood flow speed, and take an increase in the blood flow speed into account. In this case, the prediction section  16  predicts a time-dependent change in the pixel value based on the blood flow speed to which the difference is added. That is, the prediction section  16 , when it predicts a time-dependent change in the pixel value, adds the obtained difference to a blood flow speed Q per unit tissue in a compartment. 
     A part or the entirety of the embodiments described above can be described in the form of the following additional remarks, without being limited thereto. 
     (Additional remark 1) A simulator including a prediction section that repeats a re-simulation by a predetermined number or for a predetermined period. 
     (Additional remark 2) A simulator including a prediction section that repeats a re-simulation until a predicted duration is longer than or equal to a target duration. 
     This application claims the benefit of Japanese Patent Application No. 2017-130873, filed Jul. 4, 2017, which is hereby incorporated by reference herein in its entirety.