Abstract:
A condition on the attribute of an image for generating multi-frame image data containing a plurality of frames of images per file from image data is stored beforehand. When image data is received, multi-frame image data is generated from the received image data based on incidental information contained in the image data and the condition stored beforehand. Then, the generated multi-frame image data is archived. When a request for an image is made by a certain terminal, an application functioning on the terminal is identified, and multi-frame image data appropriate for the application is sent from among the generated multi-frame image data.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technology of creating a scanning plan for an X-ray CT system. 
     2. Description of the Related Art 
     An X-ray CT system may irradiate a subject with X-rays from a plurality of directions and process the respective X-rays transmitted through the subject as projection data, thereby reconstructing the inside of the subject as an image. In order to acquire and reconstruct an image of the inside of a subject with this X-ray CT system, there is a need to create a scanning plan beforehand. A scanning plan contains various kinds of setting contents, for example, scanning conditions such as a scanning region irradiated with an X-ray, a tube voltage and tube current of an X-ray tube and a time to irradiate with an X-ray, and scanning methods such as dynamic scanning and helical scanning. The scanning plan data containing such setting contents is outputted to a controller, whereby driving of the X-ray CT system is controlled. 
     Conventionally, setting of the scanning region and the tube current in the scanning plan is done based on a scout image as shown in, for example, Japanese Unexamined Patent Application Publication No. 2004-298247. A scout image is an X-ray transmission image of a subject to be captured. The X-ray CT system captures scout images from two orthogonal directions, i.e., from the front and side of a subject by dual scanogram imaging beforehand, and causes a display device to display these scout images. An operator inputs the settings of the scanning region and the tube current based on the scout images via a graphical user interface (GUI). 
     For example, when inputting the scanning region, the operator operates a mouse, a trackball, etc., to draw a desired region as the scanning region on the scout image. A frame body surrounding the region is displayed on the scout image as position mark information that represents the scanning region. 
     Further, the X-ray CT system acquires the pixel value of the scout image and, from that pixel value, calculates the tube current at a position of the X-ray tube corresponding to a view angle orthogonal to the scout image. View angles other than the view angle orthogonal to the scout image are estimated on the basis of the tube currents obtained based on the scout images from the two orthogonal directions on the assumption that the subject is an ellipse. 
     Thus, according to the technique of creating a scanning plan with a scout image, the operator needs to estimate the position, range and shape of a subject to be captured such as an organ on the basis of an X-ray transmission image of a single plane or X-ray transmission images of two orthogonal planes, and input a scanning region. It is required that the scanning region accurately includes the subject to be captured in order to obtain a high diagnostic effect, and it is important that the other regions are excluded in order to reduce unnecessary exposure. However, by the conventional method based on the estimation of the operator, it is difficult to precisely set the scanning region. 
     Further, as described above, in a case where the tube current is calculated at each position on the body axis and at each view angle from an X-ray transmission image of a single plane or X-ray transmission images of two orthogonal planes, the view angles other than the view angle orthogonal to the X-ray transmission image must be approximate values based on values calculated from the X-ray transmission image. 
     SUMMARY OF THE INVENTION 
     The present invention is devised in view of the problems as described above, and an object of the present invention is to provide an X-ray CT system capable of creating a scanning plan with high precision and also provide a method for creating a scanning plan for the system. 
     In a first aspect of the present invention, an X-ray CT system that irradiates with an X-ray to capture a cross-sectional image of a subject placed on a bed, is provided with: an image storage configured to store a previous image containing previous three-dimensional volume data of the subject; a capturing part including an X-ray tube irradiating with an X-ray, and configured to capture an X-ray transmission image of the subject; a measuring part configured to measure an amount of displacement between images of the subject shown in the previous image and the X-ray transmission image; an inputting part for setting a scanning region; a display controller configured to control so as to display position mark information representing the scanning region on an image based on the three-dimensional volume data in response to an input to the inputting part; and a scanning controller configured to move relative positions of the X-ray tube and the subject by using the amount of displacement so that the scanning region set on the image based on the three-dimensional volume data is captured, and to control the capturing part to capture the cross-sectional image. 
     Further, in a second aspect of the present invention, the X-ray CT system is further provided with: a tube current calculator configured to calculate a tube current at each capturing position on the basis of an inputted image SD value and the three-dimensional volume data, and link the each position with the tube current; and a correcting part configured to correct the position by the amount of displacement in the link, and when the X-ray tube reaches the corrected position, a tube current linked with the corrected position is applied to the X-ray tube. 
     Further, in a third aspect of the present invention, a method for creating a scanning plan for an X-ray CT system having a capturing part that includes an X-ray tube irradiating with an X-ray and that is configured to capture a cross-sectional image of a subject placed on a bed, and an inputting part for setting a scanning region of the subject, includes: storing a previous image containing previous three-dimensional volume data of the subject; capturing an X-ray transmission image of the subject by the capturing part; measuring the amount of displacement in image of the subject shown in the previous image and the X-ray transmission image; displaying position mark information showing the scanning region in response to an input to the inputting part, on an image based on the three-dimensional volume data; and moving relative positions of the X-ray tube and the subject by using the amount of displacement so that the scanning region set on the image based on the three-dimensional volume data is captured, and controlling the capturing part to capture the cross-sectional image. 
     Further, in a fourth aspect of the present invention, in the scanning plan creating method: a tube current at each capturing position is calculated based on an inputted image SD value and the three-dimensional volume data, and the each position is linked with the tube current; in the link, the position is corrected by the amount of displacement; and when the X-ray tube reaches the corrected position, a tube current linked with the corrected position is applied to the X-ray tube. 
     According to the first and third aspects, by measuring the amount of displacement of a subject from a previous image and an X-ray transmission image captured for actual scan this time and using for correction, it becomes possible to input a scanning region by using an image based on previous three-dimensional volume data of the subject, and accurately present a capturing site such as an organ to the operator as compared with a case of using a scanning plan based on an X-ray transmission image. Accordingly, it is no longer necessary for the operator to estimate the spatial position, and it is possible to determine the scanning region with high precision. 
     Further, according to the second and fourth aspects of the present invention, by referring to the three-dimensional volume data, it is also possible to precisely set a tube current for each view angle not orthogonal to a scout image, without requiring estimation, and it becomes possible to capture a favorable image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an in-hospital network that contains an X-ray CT system. 
         FIG. 2  shows the configuration of the X-ray CT system. 
         FIG. 3  shows a more detailed configuration relating to creation of a scanning plan of a main controller. 
         FIG. 4  is a flowchart showing a process of measuring the amount of displacement by the main controller. 
         FIG. 5  shows the data structure of a database for searching a previous image. 
         FIG. 6  shows a state in which a scout image of a YZ plane captured previously and a scout image of the YZ plane captured this time are superimposed. 
         FIG. 7  shows a state in which a scout image of an XZ plane captured previously and a scout image of the XZ plane captured this time are superimposed. 
         FIG. 8  is a flowchart showing a process of correcting a scanning region by the amount of displacement. 
         FIG. 9  is a schematic view showing an image based on displayed three-dimensional volume data. 
         FIG. 10  shows a scout image of the YZ plane captured this time. 
         FIG. 11  shows a scout image of the XZ plane captured this time. 
         FIG. 12  shows a detailed configuration of a main controller that automatically inputs a scanning region into an image based on three-dimensional volume data. 
         FIG. 13  is a flowchart showing a process of automatically inputting a scanning region into an image based on three-dimensional volume data. 
         FIG. 14  shows an aspect of designating a single point in the image based on the three-dimensional volume data. 
         FIG. 15  shows an aspect of extracting a region by region expansion from the designated single point in the image based on the three-dimensional volume data. 
         FIG. 16  shows an aspect of generating a scanning region including the extracted region in the image based on the three-dimensional volume data. 
         FIG. 17  shows a detailed configuration of the main controller that sets a tube current from the three-dimensional volume data. 
         FIG. 18  is a flowchart showing the process of setting the tube current from the three-dimensional volume data. 
         FIG. 19  shows an aspect of projecting the three-dimensional volume data onto a plane orthogonal to a line entering from a view angle            .
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Below, preferred embodiments of an X-ray CT system and a method for creating a scanning plan for the system according to the present invention will be specifically described with reference to the drawings. 
       FIG. 1  is a diagram showing an in-hospital network containing an X-ray CT system  10  that implements a method for creating a scanning plan according to the present embodiment. To an in-hospital network N, an image archive apparatus  1  and the X-ray CT system  10  are connected. The image archive apparatus  1  archives previous images of a subject P captured with the X-ray CT system  10 . This image archive apparatus  1  may be mounted on the X-ray CT system  10  and functionally connected therein electrically or by execution of a program of a computer. 
     The previous images archived by the image archive apparatus  1  are X-ray transmission images and three-dimensional volume data. An X-ray transmission image is a transmission image captured beforehand in pre-scan by an X-ray for creation of a scanning plan, and is also called a scout image. Three-dimensional volume data is volume data captured and obtained three-dimensionally during actual scan in accordance with a scanning plan. The previous image may be, instead of the three-dimensional volume data, only a three-dimensional image obtained by executing a rendering process on the volume data or an multiplanar reformation (MPR) image. 
     The X-ray CT system  10  creates a scanning plan with reference to images based on previous three-dimensional volume data read out from the image archive apparatus  1 . The images based on the three-dimensional volume data are three-dimensional images generated by volume rendering and MPR images of cross-sections of arbitrary sites. 
     To be specific, with reference to the images based on the previous three-dimensional volume data, a scanning region to become an X-ray irradiation region in actual scan can be inputted. Moreover, with reference to the previous three-dimensional volume data, a tube current at each view angle is set. Specifically, the X-ray CT system  10  displays an image based on the previous three-dimensional volume data read out from the image archive apparatus  1  in order to create a scanning plan. The operator refers to the image based on the previous three-dimensional volume data, and inputs the range of a scanning region irradiated with an X-ray during actual scan in relationship with the subject P shown in the image based on the previous three-dimensional volume data. For example, in a case where capture of the left lung field of the subject P is aimed, the range of the scanning region is defined so as to encompass the left lung field of the subject P shown in the image based on the three-dimensional volume data. The range of the scanning region to be inputted is a position and a spread with reference to the position. 
     In actual scan, a bed for placing the subject is held at one end of the scanning region or backward by an extra width from the position, and then moved to the other end of the scanning region, whereby the set scanning region is irradiated with an X-ray. Accordingly, by calculating a bed position from the coordinate position of an image reflecting the present placement state of the subject P on the bed, the scanning region is eventually represented with information showing the bed position. The on-bed placement state includes the placement positions of the subject P in the long-side direction and the short-side direction of the bed, and the height of the bed. 
     There is a difference in on-bed placement state of the subject between the time of capture of the previous three-dimensional volume data and the present time. Therefore, the X-ray CT system  10  measures the amount of displacement in image of the subject P between the time of capture of the previous image and the time of capture of the scout image this time, from the previous image and the scout image captured in pre-scan this time. Then, the position of the scanning region inputted in relationship with the image based on the previous three-dimensional volume data is corrected by the amount of displacement. 
     Consequently, even when the image based on the previous three-dimensional volume data is referred to, the scanning region reflecting the present on-bed placement state of the subject P is set. 
     The previous image for measuring the amount of displacement may be an image based on previous three-dimensional volume data, or may be a previous scout image captured in pre-scan for capturing the previous three-dimensional volume data. In the present embodiment, the description is made by using a scout image with which measurement of the amount of displacement is easy. In a case where an image based on three-dimensional volume data is used as a previous image for measuring the amount of displacement, data of the same plane as the scout image is extracted from the three-dimensional volume data, and an image represented by the data is used. Moreover, by using two types of scout images captured from two orthogonal directions, it is possible to measure the amount of displacement in each direction of three dimensions and obtain the accurate amount of displacement. Therefore, the present embodiment is described on the premise of dual scanograms. However, it is also possible to measure the amount of displacement by using a scout image of only a single direction, and it is possible to create a scanning plan with reference to an image based on previous three-dimensional volume data. 
       FIG. 2  is a diagram showing the configuration of the X-ray CT system  10 . This X-ray CT system  10  continuously changes the X-ray irradiation position around the body axis of the subject P or in the direction of the body axis to collect projection data reflecting the transmission amount of the X-ray from the subject P and reconstruct the projection data, thereby acquiring data that represents the inside of the body of the subject P. 
     The X-ray CT system  10  includes a mount  12 , a bed  14 , and a processing unit  16 . The mount  12  is a capturing part that irradiates the subject P with X-rays from a plurality of directions and detects the X-ray transmitted through the subject P. Moreover, the mount  12  also serves as a capturing part that captures a scout image of the subject P by irradiating the subject P with an X-ray from a single direction to the body axis direction and detecting the X-ray transmitted through the subject P. 
     This mount  12  has an aperture  18  into which the subject P is inserted. The bed  14  is a device that inserts the subject P into the aperture  18 . The processing unit  16  is a device that generates scanning plan data for controlling the drive of the mount  12  and the bed  14  to integrally control the mount  12  and the bed  14 , and executes an image reconstruction process on X-ray transmission data, thereby generating and displaying an image of the inside of the subject P. The scanning plan data is sequence data representing a scanning plan that defines at least one of the scanning region and the tube current or both of them. 
     The mount  12  houses a gantry  20  and a rotation drive  22  therein. 
     The gantry  20  is a ring body that can rotate about the aperture  18 . The rotation drive  22  is composed of a motor and a gear or the like that has an interlocking relationship with the gantry  20 , and rotates the gantry  20  about the aperture  18 . 
     On the gantry  20 , an X-ray tube  24  and an X-ray detector  26  are installed facing each other across the aperture  18 . Moreover, on the gantry  20 , a collimator  28  is disposed between the X-ray tube  24  and the X-ray detector  26 . Inside the mount  12 , a high-voltage generator  30  is disposed in pair with the X-ray tube  24 , a diaphragm drive  32  is disposed in pair with the collimator  28 , and a data acquisition system  34 , which is called DAS (data acquisition system), is disposed in pair with the X-ray detector  26 . 
     Further, the mount  12  is provided with a mount controller  36 , which is a scanning controller on the side of the mount  12 . This mount controller  36  controls the high-voltage generator  30 , the diaphragm drive  32 , the data acquisition system  34  and the bed  14  in accordance with the scanning plan data. That is, the mount controller  36  sends a control signal for controlling a tube current to the high-voltage generator  30 , in accordance with information representing the tube current contained in the scanning plan data. The mount controller sends a control signal for controlling an irradiation field to the diaphragm drive  32 , in accordance with information representing the range of the irradiation field contained in the scanning plan data. The mount controller sends control signals for controlling a movement starting position, a movement velocity and a movement ending position to the bed  14 , in accordance with information representing the scanning region contained in the scanning plan data. 
     The high-voltage generator  30  supplies an electric current for heating a filament to the X-ray tube  24  and applies a high voltage. As the high-voltage generator  30 , a high-frequency inverter type is applied, which is a type of rectifying alternating current power of 50/60 Hz into a direct current, converting it to a high-frequency alternating current of several kHz or more to boost the pressure, and rectifying it again to apply it. The X-ray tube  24  receives the supply of the electric current and the application of the high voltage and generates an X-ray. The collimator  28  is driven by the diaphragm drive  32  to define the irradiation field of the X-ray and block X-rays outside the irradiation field, thereby narrowing the X-ray generated by the X-ray tube  24  down to a fan-shaped or cone-shaped beam. The X-ray detector  26  is provided with multivariate and multichannel X-ray detecting elements arrayed in two orthogonal directions. The array shape is an arc shape around the focal point of the X-ray generated by the X-ray tube  24 . The mainstream X-ray detecting element is an indirect conversion type that converts an X-ray into light by a phosphor such as a scintillator and further converts the light into a charge by a photoelectric conversion element such as a photodiode, and a direct conversion type using generation of an electron-hole pair in a semiconductor by an X-ray and movement thereof to an electrode, namely, using a photoconductive phenomenon. This X-ray detector  26  detects the X-ray transmitted through the subject P, and outputs an X-ray transmission signal reflecting the transmission amount of the detected X-ray, for each of the X-ray detecting elements. The data acquisition system  34  collects the X-ray transmission signals from the respective X-ray detecting elements every time the control signal is inputted from the mount controller  36 . For each of the radiation detecting elements, an I-V converter, an integrator, a preamplifier and an A/D converter are provided to convert a current signal from each of the radiation detecting elements into a voltage signal, integrate the voltage signal in synchronization with the radiation exposure period, amplify, and convert into a digital signal. The data acquisition system  34  outputs the X-ray transmission signals converted into digital signals to the processing unit  16 . The X-ray transmission signals outputted from the data acquisition system  34  are digital data containing the value of the integer of each absorption coefficient along the transmission length of the X-ray sequentially transmitted through materials with different X-ray absorption coefficients. It is so-called raw data. 
     The bed  14  is provided with a top board  38  for placing the subject P and a bed driving part  40 . The top board  38  is driven by the bed driving part  40  to slide in a long-side direction (a Z-axis direction in the drawing) to be inserted into the aperture  18 . In the processing unit  16 , a preprocessor  42 , a projection data storage  44 , a reconstruction processor  46 , an image storage  48 , an image processor  50 , and a display device  52  are sequentially connected and implemented. Moreover, a main controller  54 , an input device  56 , and a network interface  58  are disposed in the processing unit  16 . 
     The preprocessor  42  executes sensitivity correction on raw data. 
     The raw data having been subjected to the sensitivity correction is called projection data, which is inputted and stored in the projection data storage  44 . The reconstruction processor  46  mainly uses a reconstruction algorithm called the Feldkamp method to read out the corrected projection data from the projection data storage  44 , followed by back projection, and reconstructs the inside of the subject P as image data. The reconstructed image data is inputted and stored in the image storage  48 . The image processor  50  subjects image data stored in the image storage  48  to a variety of image processing such as scan conversion of converting into a video format of the orthogonal coordinate system to generate a display image. The display device  52  is a monitor such as a liquid crystal display or a CRT display, which displays a display image that has been generated by the image processor  50 . The main controller  54  is a scanning controller on the console side that generates scanning plan data according to operation via the input device  56  and outputs it to the mount controller  36 . The input device  56  is a pointing device such as a trackball or a mouse having a click button that moves a cursor displayed on the display device  52  for selecting a button or menu option via a GUI, or a keyboard that inputs symbol strings such as numbers and characters. 
     The network interface  58  constitutes equipment such as a LAN card, a LAN board, or a LAN adaptor comprising a connector for connecting the cable of a network adaptor, and a circuit required for connecting to the network in compliance to LAN standards such as Ethernet. 
       FIG. 3  is a block diagram showing a more detailed configuration relating to creation of a scanning plan of the main controller  54 . The main controller  54  has a previous-image search receiver  60 , a previous-image storage  62 , a display controller  66 , a rendering processor  64 , a correcting part  68 , and a scanning plan data generator  70 . The respective parts may be composed of dedicated circuits, or may be functionally achieved through execution of a program by a CPU. 
     The previous-image search receiver  60  causes the image archive apparatus  1  to search a previous image of the subject P, and receives the previous image of the subject P from the image archive apparatus  1 . 
     The previous-image search receiver  60  sends a search key that specifies the subject P inputted by using the input device  56  to the image archive apparatus  1 , thereby causing the image archive apparatus  1  to search. The transmission of the search key and the reception of the previous image are conducted via the network interface  58  and the network N. The received previous image is stored in the previous-image storage  62 . 
     Among the previous images of the subject P, the display controller  66  reads out a three-dimensional image based on three-dimensional volume data and a cross-sectional image (an MPR image cut into round slices along the XY plane in the drawing) from the previous-image storage  62 , and controls the display device  52  to display them. Moreover, in response to an operation of inputting the scanning region with the input device  56 , the display controller  66  controls to display position mark information that represents the scanning region within the three-dimensional image and on the cross-sectional image. The position mark information displayed within the three-dimensional image is represented with a circular cylindrical frame body, and the position mark information displayed on the cross-sectional image is represented with a circular frame body. When the operator moves the scanning region or performs an operation of enlarging or shrinking it by using the input device  56 , the display controller  66  controls to move, or enlarge or shrink the position mark information within the three-dimensional image and on the cross-sectional images, in response to the input operation. 
     The rendering processor  64  executes a volume rendering process on the three-dimensional volume data stored in the previous-image storage  62  to generate a three-dimensional image and a cross-sectional image. In a case where the previous-image storage  62  stores no three-dimensional image or cross-sectional image, i.e., in a case where the image archive apparatus  1  stores no three-dimensional image and stores only three-dimensional volume data and the previous-image search receiver  60  receives a scout image and three-dimensional volume data alone as a previous image, volume rendering is performed. 
     The correcting part  68  corrects the position of the scanning region inputted with the input device  56  so as to match the on-bed placement state of the subject P for scan this time. First, from a scout image captured beforehand for generating scanning plan data of actual scan this time and a scout image stored in the previous-image storage  62 , the correcting part  68  calculates the amount of displacement of the subject shown in the scout images. Then, the correcting part shifts the position of the scanning region inputted with the input device  56  by this displacement amount. 
     The scanning plan data generator  70  generates scanning plan data containing information showing the corrected scanning region, and sends it to the mount controller  36 . 
       FIG. 4  is a flowchart showing a process of measuring the amount of displacement by the main controller  54 . 
     First, the subject P is placed on the top board  38 , and a scout image of the placed subject P is captured (S 01 ). In capturing the scout image, the main controller  54  first generates scanning plan data for creation of scout images from two orthogonal directions, such as the front, i.e., the YZ plane, and the side, i.e., XZ plane. The main controller  54  outputs this created scanning plane data to the mount controller  36 . The mount controller  36  starts control to capture a scout image of the subject P placed on the top board  38  in accordance with the inputted scanning plan data. The mount controller  36  into which the scanning plan data for scout image creation has been inputted outputs a control signal to the rotation drive  22 , and rotates and fixes the X-ray tube  24  so as to face the front of the subject P, i.e., the YZ plane. Then, while a control signal for rotation is not outputted to the rotation drive  22  that rotates the gantry  20 , drive signals are outputted to the high-voltage generator  30 , the diaphragm drive  32 , the data acquisition system  34 , and the bed-driving part  40 . As the X-ray tube  24  and the X-ray detector  26  are fixed in front of the subject P, the top board  38  is moved in the Z-axis direction and irradiated with an X-ray, whereby the front of the subject P is irradiated with the X-ray along the body axis direction, and a scout image of the YZ plane is captured from the front of the subject P. Next, the mount controller  36  returns the top board  38  to the origin position, outputs a control signal to the rotation drive  22 , and rotates the X-ray tube  24  by 90 degrees to fix so as to face the side of the subject P, i.e., the XZ plane. Then, the top board  38  is moved in the Z-axis direction and irradiated with an X-ray, whereby the side of the subject P is irradiated with the X-ray along the body axis direction, and a scout image of the XZ plane is captured from the side of the subject P. The scout images of the YZ plane and the XZ plane of the subject P are collected by the data acquisition system  34  and stored in the image storage  48 . 
     When information that specifies the subject P is inputted with the input device  56  around the time when the scout images are captured (S 02 ), the previous-image search receiver  60  sends a search command with the information specifying the subject P as a search key to the image archive apparatus  1  via the network interface  58  and the network N (S 03 ). Then, a previous image of the subject P is received from the image archive apparatus  1  (S 04 ). The information specifying the subject P is, for example, a patient ID or a patient&#39;s name. The previous-image search receiver  60  causes the previous-image storage  62  to store the received previous image. 
       FIG. 5  is a data configuration view showing a database for searching a previous image. As shown in  FIG. 5 , the image archive apparatus  1  stores subject information that specifies the subject P, capturing date information that represents the date when a previous image is captured, site information that represents a site where a previous image is captured, and archive destination information that represents the destination to archive a previous image, so as to be linked with each other. When receiving information that specifies the subject P, the image archive apparatus  1  searches a record containing the information specifying the subject P from the database, and sends a list of the records containing the information specifying the subject P to the X-ray CT system  10 . When receiving the list of the records, the previous-image search receiver  60  causes the display device  52  to display this list. When one of the records is selected from the list with the input device  56 , the previous-image search receiver  60  sends a request for sending the previous image that is the main constituent of this record. When receiving this sending request, the image archive apparatus  1  reads out the archive destination information of the previous image from the requested record, and sends the previous image stored in the storage destination to the X-ray CT system  10 . 
     After the scout image is captured and the previous image is received, the correcting part  68  reads out a scout image of the YZ plane captured at S 01  and a scout image of the YZ plane contained in the previous image from the image storage  48  and the previous-image storage  62  (S 05 ), superimposes the two scout images with each other (S 06 ), and measures the amount of displacement in the Y-axis direction and Z-axis direction (Ygap, Zgap) of the images of the subject P shown in both the scout images (S 07 ). 
       FIG. 6  is a view showing a state in which a previous scout image of the YZ plane and a scout image of the YZ plane captured this time are superimposed. The two scout images are superimposed so that reference points Bp coincide. As the reference points Bp, the correcting part  68  makes the origins of the two images coincide. In general, an image is captured so that a specific point on the top board  38  corresponds to the origin position of the image. Because an on-bed placement state at the time of capture of a previous scout image is different from an on-bed placement state at the time of capture of a scout image this time, displacement occurs in the images of the subject P when they are superimposed so that the reference points Bp coincide. 
     The correcting part  68  extracts a portion having a characteristic shape easy to extract, such as the left collarbone portion, as a common site Cp from each of the scout images. Then, a coordinate difference Ygap in the Y-axis direction and a coordinate difference Zgap in the Z-axis direction of both pixels composing the left collarbone portion are measured. The measured value is stored as the amount of displacement in the Y-axis and Z-axis directions (Ygap, Zgap). 
     Furthermore, the correcting part  68  reads out the captured scout image of the XZ plane and the scout image of the XZ plane contained in the previous image (S 08 ), superimposes the two scout images (S 09 ), and measures the amount of displacement Xgap in the X-axis direction of the subject P shown in both the scout images (S 10 ). 
       FIG. 7  is a view showing a state in which a previous scout image of the XZ plane and a scout image of the XZ plane captured this time are superimposed. The two scout images are superimposed so that the reference points Bp coincide. As the reference points Bp, the correcting part  68  makes the origins of both the images coincide. The correcting part  68  extracts a portion having a characteristic shape easy to extract, such as the left collarbone portion, as the common site Cp from each of the scout images, and measures a coordinate difference Xgap on the X-axis in both pixels composing the left collarbone portion. This measured value is stored as the amount of displacement Xgap in the X-axis direction. 
     Next, a process of correcting the scanning region inputted into the previous three-dimensional image on the basis of the obtained amount of displacement (Xgap, Ygap, Zgap) so as to reflect the current on-bed placement state of the subject P will be described with reference to  FIG. 8 .  FIG. 8  is a flowchart showing the process of correcting the scanning region by the amount of displacement. First, the rendering part  64  searches whether any three-dimensional image of the subject P exists in the previous-image storage  62  (S 11 ). In a case where no three-dimensional image exists in the previous-image storage  62  (S 11 , No), the rendering part  64  reads out three-dimensional volume data of the subject P from the previous-image storage  62  (S 12 ), and generates a three-dimensional image by performing volume rendering (S 13 ). In a case where the previous-image storage  62  stores no cross-sectional image of the subject P as well, the rendering part  64  also generates a cross-sectional image by executing an MPR process on the three-dimensional volume data. 
     In a case where a three-dimensional image exists in the previous-image storage  62  (S 11 , Yes), or when a three-dimensional image is generated by the rendering part  64  (S 13 ), the display controller  66  reads out a three-dimensional image and a cross-sectional image from the previous-image storage  62  (S 14 ), and generates a display image in which the three-dimensional image and the cross-sectional image are put side by side to control the display device  52  to display them (S 15 ). When an operation of inputting the scanning region is executed with the input device  56  (S 16 ), the display controller  66  generates a display image in which position mark information representing the scanning region is synthesized on the three-dimensional image and the cross-sectional image to cause the display device  52  to display them (S 17 ). 
       FIG. 9  is a schematic view showing a displayed three-dimensional image. A previous cross-sectional image is displayed side by side with a previous three-dimensional image. On the three-dimensional image, a cylindrical frame body representing a scanning region R is displayed as position mark information. This scanning region R is displayed by moving a cursor C on the screen to one corner of a site desired to input as the scanning region R by a mouse or a trackball of the input device  56  and moving the cursor C while dragging it to the other corner. The display controller  66  generates a display image in which the starting and ending points are included in the outer shell and a cylindrical frame body having the center of gravity at the midpoint between the starting point and the ending point is synthesized in the image based on the three-dimensional volume, and controls the display device  52  to display the display image. For example, when desiring to set a region containing the left lung field of the subject P as the scanning region R, the operator operates the input device  56  so as to encompass the left lung field. The display controller  66  synthesizes the cylindrical frame body encompassing the left lung field into the image based on the three-dimensional volume, and controls the display device  52  to display it. 
     Further, the display controller  66  synthesizes a circular or square frame body representing the range of the scanning region R contained in a cross-sectional image including one end face of the scanning region R, generates a display image in which the frame body is put with the three-dimensional image, and controls the display device  52  to display the display image. Moreover, a button (not shown) for turning cross-sectional images is displayed on the screen. When this button is pressed down, the display controller  66  reads out a next cross-sectional image closer to the other end face of the scanning region R than the currently displayed cross-sectional image from the previous-image storage  62 , superimposes the frame body representing the range of the scanning region R contained in that cross-sectional image, and generates a display image in which the frame body is put with the three-dimensional image, thereby controlling the display device  52  to display the display image. 
     When the scanning region is displayed in the image based on the three-dimensional volume by the display controller  66 , the correcting part  68  acquires the coordinate range (X, Y, Z) of the scanning region in the image based on the three-dimensional volume (S 18 ). For example, when the scanning region is set so as to encompass the left lung field of the subject P, the correcting part  68  acquires the coordinate range on the three-dimensional volume data coordinate system of the frame body encompassing the left lung field. 
     When the coordinate range (X, Y, Z) of the scanning region in the three-dimensional volume data is acquired, the correcting part  68  reads out the amount of displacement (Xgap, Ygap, Zgap) (S 19 ), and subtracts the amount of displacement (Xgap, Ygap, Zgap) from the acquired coordinate range (X, Y, Z) of the scanning region, thereby correcting the scanning region on the coordinate system of the scout image captured this time. Consequently, the coordinate range (X-Xgap, Y-Ygap, Z-Zgap) of the scanning region reflecting the current on-bed placement state is acquired (S 20 ). The scanning plan data generator  70  generates scanning plan data containing the coordinate range (X-Xgap, Y-Ygap, Z-Zgap) of the corrected scanning region (S 21 ), and outputs it to the mount controller  36  (S 22 ). 
       FIG. 10  and  FIG. 11  are schematic views showing scout images captured this time.  FIG. 10  shows a scout image of the YZ plane, and  FIG. 11  shows a scout image of the XZ plane. The scout images captured this time represent the current on-bed placement state of the subject P. For example, the scanning region R is matched with the left lung field of the subject P shown in the image based on the previous three-dimensional volume, and the coordinate range (X, Y, Z) in the three-dimensional volume data of this scanning region R is applied to the scout image captured this time without modification. Then, the left lung field of the subject P in the current on-bed placement state of is not included in the applied scanning region. 
     However, when the correcting part  68  shifts the coordinate range (X, Y, Z) in the three-dimensional volume data by the measured amount of displacement (Xgap, Ygap, Zgap), the left lung field of the subject P in the current on-bed placement state is included in the range indicated by the coordinate range of a corrected scanning region RN (X-Xgap, Y-Ygap, Z-Zgap) on the scout image that has been captured this time. By calculating the bed position from the coordinate range showing the corrected scanning region RN (X-Xgap, Y-Ygap, Z-Zgap), even if the scanning region R is set using the image based on the past three-dimensional image, the bed  14  may be driven according to the current state of being placed on the bed for defining the irradiation field. 
     Thus, in the X-ray CT system  10  according to the present embodiment, the amount of displacement (Xgap, Ygap, Zgap) of the subject P between the previous image and the current scout image is measured beforehand and, when the scanning region is inputted in an image based on the previous three-dimensional volume, the coordinate range (X, Y, Z) of this scanning region is corrected by the amount of displacement (Xgap, Ygap, Zgap). Then, actual scanning of irradiating the scanning region after the position correction (X-Xgap, Y-Ygap, Z-Zgap) with an X-ray is performed. Consequently, the scanning region can be inputted by using the image based on the previous three-dimensional volume of the subject P, whereby it is possible to present a capturing site such as an organ to the operator more accurately than with a scanning plan based on an X-ray transmission image. 
     Accordingly, it is no longer necessary for the operator to estimate the spatial position, and the scanning region can be determined with precise accuracy. 
     The above description is based on dual scanograms, but as previously described, it is possible to capture only a scout image of the front of the subject P, i.e., the YZ plane, and calculate the amount of displacement (Xgap, Ygap, Zgap). The amount of displacement in the X direction, i.e., in the direction of the bed height, depends on the height of the bed  14 , because the height of the subject P from the height above the top board of the bed  14  will not change. Information that represents the height of the bed  14  is incidental to an image in the DICOM standard. Therefore, the correcting part  68  calculates the amount of displacement Xgap on the basis of the inputted height of the bed  14  and the height of the bed  14  that is incidental to a previous image. 
     Further, an aspect of inputting the range of the scanning region to an image based on the previous three-dimensional volume data is described above. However, it is also possible to configure so that the shape of an organ shown in an image based on three-dimensional volume data is recognized and a scanning region matched to the organ is automatically inputted within the image based on the three-dimensional volume data. 
       FIG. 12  is a block diagram showing a detailed configuration of the main controller  54  that automatically inputs the scanning region into the image based on the three-dimensional volume data. The same configuration will be provided with the same name and the same reference numeral, and a detailed explanation thereof will be omitted. 
     In the X-ray CT system  10  that automatically inputs the scanning region into the image based on the three-dimensional volume data, the main controller  54  further has a CT-value acquiring part  78  and an extracting part  80 , in addition to the display controller  66 . 
     When a single point within an image based on three-dimensional volume data is designated, the CT-value acquiring part  78  acquires the CT value of the designated position. The CT value represents an X-ray absorption coefficient as a relative value with reference to water in general. This CT value is expressed by [(μ−μ 0 )/μ 0 ]×K, wherein μ is an absorption coefficient of a material, μ 0  is an absorption coefficient of a reference material and K is a constant. In general, the CT value of water is 0, and K is equal to 1,000, so the CT value of air is −1,000. In three-dimensional volume data, a pixel value of a pixel corresponds to the CT value. That is, the CT-value acquiring part  78  acquires the CT value of the designated position. The single point within the image based on the three-dimensional volume data is designated through an operation with the input device  56 . When a trackball or a mouse of the input device  56  is operated to move a cursor within a display screen of the display device  52  to a designated position and is clicked, the CT value of the pixel corresponding to the clicked coordinate position is read out. 
     The extracting part  80  extracts the outline of a tissue such as an organ containing the designated single point within the image based on the three-dimensional volume data. The collection of pixels having substantially the same CT value as the CT value of the designated single point within the image based on three-dimensional volume data is extracted by the region expansion method, and the coordinate of each of the pixels composing the outline of the region. The range of substantially the same CT value is a predetermined range centered on the CT value of the designated single point. This range is stored beforehand in an internal ROM of the extracting part  80 . 
     The display controller  66  generates a scanning region that includes the region extracted by the extracting part  80 , and synthesizes it into the image based on the three-dimensional volume data to control the display device  52  to display. In the main controller  54 , the correcting part  68  corrects the position of the scanning region that includes the extracted region extracted by the extracting part  80 , and the scanning plan data generator  70  makes the scanning region after the position correction contained in the scanning plan data to be sent to the mount controller  36 . 
       FIG. 13  is a flowchart showing a process of automatically inputting the scanning region into the image based on the three-dimensional volume. First, as shown in  FIG. 14 , when a single point Po within the image based on the three-dimensional volume data is designated with the input device  56  (S 31 ), the CT-value acquiring part  78  acquires the CT value of the designated point Po within the image based on the three-dimensional volume data (S 32 ). 
     When the CT value of the designated single point Po is acquired, the extracting part  80  extracts a region that contains the designated single point Po and has substantially the same CT value as the CT value of the designated single point Po (S 33 ). As shown in  FIG. 15 , while expanding a search region around the designated single point, the extracting part  80  compares the CT value of a search target pixel with the acquired CT value. Then, in a case where the CT value of the search target pixel is included in the predetermined range where the CT value is substantially the same as the acquired CT value, this search target pixel is included in an extracted region Ra. Extraction ends if all of the search target pixels have the CT values outside the predetermined range after expansion of the search region. 
     When the region is extracted, as shown in  FIG. 16 , the display controller  66  generates the scanning region R that includes this extracted region Ra (S 34 ), and synthesizes it with the image based on the three-dimensional volume data to control the display device  52  to display (S 53 ). 
     Thus, as a scanning plan with reference to the previous three-dimensional volume data can be developed, extraction of a tissue image on an image, which is difficult in an X-ray transmission image, becomes possible, and setting of the scanning region becomes simple and accurate. In addition, by referring to the three-dimensional volume data, it is possible to accurately set the tube current at each view angle. 
       FIG. 17  is a block diagram showing a detailed configuration of the main controller  54  configured to set the tube current from the three-dimensional volume data. The same configuration will be provided with the same name and the same reference numeral, and a detailed explanation thereof will be omitted. The main controller  54  further has an X-ray transmission image generator  72 , a tube current pattern storage  74 , and a tube current calculator  76 . 
     The X-ray transmission image generator  72  generates each X-ray transmission image projected on a plane orthogonal to a line entering from each view angle, on the basis of the three-dimensional volume data. The tube current pattern storage  74  archives information in which the pattern of a pixel value and a tube current value is set for each image SD value. The tube current pattern is acquired beforehand by using, for example, a human body or a human body simulated phantom. 
     The image SD is a numerical value that represents image noise defining the variation in pixel values of a homogeneous phantom image as display deviation. 
     The tube current calculator  76  acquires the pixel value of the X-ray transmission image generated by the X-ray transmission image generator  72  and, from the pattern of the pixel value and the tube current value, calculates the tube current supplied by the high-voltage generating device  30  when the X-ray tube  24  is located at the coordinate position (Z,            ) that passes through the pixel with the acquired pixel value, the view angle θ being orthogonal to the X-ray transmission image. Of the coordinate position (Z,          ) of the X-ray tube  24 , the coordinate value of the Z-axis corresponds to the coordinate system of the previous three-dimensional volume data.
     The correcting part  68  corrects the coordinate position (Z,            ) of the X-ray tube  24  by the amount of displacement Zgap in the Z-axis direction, and sets the calculated tube current as the value when the X-ray tube  24  is located at the coordinate position (Z-Zgap,          ) in the scout image captured this time. The scanning plan data generator  70  generates scanning plan data containing the tube current when the X-ray tube  24  is located at the coordinate position (Z-Zgap,          ), and outputs it to the mount controller  36 .
       FIG. 18  is a flowchart showing a process of setting the tube current by using the three-dimensional volume data. First, a pull-down menu for selecting an image SD value is displayed on an input format screen displayed for creating a scanning plan of the display device  52 , so an operation of selecting the image SD is previously inputted by the operator using the input device  56  (S 41 ). 
     As shown in  FIG. 19 , the X-ray transmission image generator  72  projects three-dimensional volume data onto a plane orthogonal to a line entering from the view angle            , thereby generating an X-ray transmission image orthogonal to the line entering from the view angle           (S 42 ).
     Next, the tube current calculator  76  acquires the pixel value of a point Z of the X-ray transmission image (S 43 ). As shown in  FIG. 19 , the pixel value of the point Z corresponding to a point on a cylinder axis in the scanning region inputted into an image based on the three-dimensional volume is acquired. This point Z is represented by a coordinate Z on the Z-axis corresponding to the point on the cylinder axis in the scanning region. This is because the X-ray is radiated so as to pass through the point on the cylinder axis from the view angle            .
     When the pixel value is acquired, the tube current calculator  76  reads out a tube current value corresponding to the previously selected image SD and the acquired pixel value, from the pattern of the pixel value and the tube current value stored in the tube current pattern storage  74  (S 44 ). When reading out the tube current value, the tube current calculator  76  links the coordinate position (Z,            ) of the X-ray tube  24  with the tube current value having been read out (S 45 ).
     The tube current calculator  76  repeats steps S 42  to S 45  if calculation and link of the tube current is not completed for all view angles             and each point Z on the cylinder axis (S 46 , No). If the calculation and link is completed all (S 46 , Yes), the correcting part  68  corrects each coordinate position (Z,          ) of the X-ray tube  24  with which the tube current is linked by the amount of displacement Zgap, thereby setting it as the coordinate position (Z-Zgap,          ) (S 47 ).
     The scanning plan data generator  70  generates scanning plan data containing link of the tube current with each coordinate position having been corrected (Z-Zgap,            ) of the X-ray tube  24  (S 48 ), and outputs it to the mount controller  36  (S 49 ).
     Accordingly, by referring to the three-dimensional volume data, it is possible to precisely set a tube current for each view angle that is not orthogonal to a scout image without the need for estimation, whereby it becomes possible to capture a favorable image.