Abstract:
The present invention is directed to realize an X-ray CT apparatus for properly performing contrast imaging. An X-ray CT apparatus includes an imaging unit and a control unit for controlling the imaging unit. The control unit includes: a first control unit for performing a monitoring scan to monitor arrival of a contrast agent at a region of interest in a start position of a main scan in an imaging range of the main scan that is set along the body axis of the subject or a first monitoring position that is set before the start position in a scan progress direction and, on arrival of the contrast agent, starting the main scan; and a second control unit for monitoring whether or not the contrast agent has reached a region of interest in a second monitoring position provided on the forward side of the start position of the main scan in the imaging range in the scan progress direction on arrival of the main scan at the second monitoring position, when the contrast agent has reached the second monitoring position, continuing the main scan and, when the contrast agent has not reached the second monitoring position yet, performing a monitoring scan for monitoring arrival of the contrast agent.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an X-ray CT (Computed Tomography) apparatus and, more particularly, to an X-ray CT apparatus for capturing an image of a subject in which a contrast agent is injected. 
         [0002]    In an X-ray CT apparatus for capturing an image of a subject in which a contrast agent is injected, prior to a main scan, a helical scan is started synchronously with arrival of the contrast agent at a region of interest (ROI) in a monitoring scan executed with a dose lower than that of the main scan. In this case, after the scan starts, the helical scan with constant progress speed is performed (refer to, for example, Japanese Unexamined Patent Publication No. Hei09 (1997)-327454). 
       SUMMARY OF THE INVENTION 
       [0003]    In an X-ray CT apparatus using a combination of a multiple-channel X-ray detector or a plane X-ray detector and a cone beam X-ray, because of increase in the helical pitch and increase in the detector width in the body axis direction due to three-dimensional image reconstruction, the progress speed of the helical scan is increasing. Consequently, there is a case such that a helical scan is faster than a contrast agent moving with blood flow, and contrast imaging cannot be performed properly. 
         [0004]    An object of the present invention is to realize an X-ray CT apparatus for properly performing contrast imaging. 
         [0005]    The present invention for solving the problem provides an X-ray CT apparatus including: an imaging unit for reconstructing an image on the basis of projection data obtained by scanning a subject in which a contrast agent is injected with an X ray; and a control unit for controlling the imaging unit. The control unit includes: a first control unit for performing a monitoring scan to monitor arrival of a contrast agent at a region of interest in a start position of a main scan in an imaging range of the main scan that is set along the body axis of the subject or a first monitoring position that is set before the start position in a scan progress direction and, on arrival of the contrast agent, starting the main scan; and a second control unit for monitoring whether or not the contrast agent has reached a region of interest in a second monitoring position provided on the forward side of the start position of the main scan in the imaging range in the scan progress direction on arrival of the main scan at the second monitoring position, when the contrast agent has reached the second monitoring position, continuing the main scan and, when the contrast agent has not reached the second monitoring position yet, performing a monitoring scan for monitoring arrival of the contrast agent. 
         [0006]    The control unit monitors arrival of the contrast agent by detecting the contrast agent by using an image obtained by reconstructing a slice image in the monitoring position. The contrast agent is detected on the basis of a CT number in the region of interest. The second control unit assigns a higher priority to the image reconstruction in the second monitoring position than image reconstruction in the other imaging range. 
         [0007]    As the second monitoring position, a plurality of monitoring positions are set at intervals in the body axis direction of the subject. A plurality of regions of interest in which arrival of the contrast agent is monitored can be set. The X-ray is a cone beam X-ray. The scan is a helical scan. 
         [0008]    According to the present invention, an X-ray CT apparatus includes: an imaging unit for reconstructing an image on the basis of projection data obtained by scanning a subject in which a contrast agent is injected with an X ray; and a control unit for controlling the imaging unit. The control unit includes: a first control unit for performing a monitoring scan to monitor arrival of a contrast agent at a region of interest in a start position of a main scan in an imaging range of the main scan that is set along the body axis of the subject or a first monitoring position that is set before the start position in a scan progress direction and, on arrival of the contrast agent, starting the main scan; and a second control unit for monitoring whether or not the contrast agent has reached a region of interest in a second monitoring position provided on the forward side of the start position of the main scan in the imaging range in the scan progress direction on arrival of the main scan at the second monitoring position, when the contrast agent has reached the second monitoring position, continuing the main scan and, when the contrast agent has not reached the second monitoring position yet, performing a monitoring scan for monitoring arrival of the contrast agent. Thus, the X-ray CT apparatus properly performing contrast imaging can be realized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a diagram showing the configuration of an X-ray CT apparatus as an example of the best mode for carrying out the invention. 
           [0010]      FIG. 2  is a diagram showing the configuration of an X-ray CT apparatus as an example of the best mode for carrying out the invention. 
           [0011]      FIG. 3  is a diagram showing the configuration of an X-ray irradiating/detecting apparatus. 
           [0012]      FIG. 4  is a diagram showing the configuration of an X-ray entrance plane of an X-ray detector. 
           [0013]      FIG. 5  is a flowchart showing operations of contrast imaging. 
           [0014]      FIG. 6  is a diagram showing an imaging range and contrast agent monitoring positions. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    A best mode for carrying out the invention will be described hereinbelow with reference to the drawings. The present invention is not limited to the best mode for carrying out the invention.  FIG. 1  shows a schematic configuration of an X-ray CT apparatus. The apparatus is an example of the best mode for carrying out the invention. By the configuration of the apparatus, an example of the best mode for carrying out the invention related to the X-ray CT apparatus will be described. By the operation of the apparatus, an example of the best mode for carrying out the invention related to a scan control method will be described. 
         [0016]    The apparatus has a gantry  100 , a table  200 , and an operator console  300 . The gantry  100  scans a subject  10  conveyed by the table  200  with an X-ray irradiating/detecting apparatus  110  to collect a plurality of views of projection data, and inputs the projection data to the operator console  300 . 
         [0017]    The operator console  300  reconstructs an image on the basis of the projection data input from the gantry  100 , and displays a reconstructed image on a display  302 . The image reconstruction is performed by a dedicated computer in the operator console  300 . The computer for image reconstruction, the gantry  100 , and the table  200  are examples of the imaging unit in the present invention. 
         [0018]    The operator console  300  controls the operation of the gantry  100  and the table  200 . The control is performed by the dedicated computer in the operator  300 . The computer includes a control unit  301  in the invention. The control unit  301  also includes a first control unit  311  and a second control unit  312  in the invention. The control unit  301  also controls the image reconstruction. 
         [0019]    Under control of the operator console  300 , the gantry  100  performs a scan with predetermined scan parameters, and the table  200  positions the subject  10  so that a predetermined region is scanned. The positioning is performed by adjusting the height of a top board  202  and the horizontal travel distance of a cradle  204  on the top board by a built-in position adjusting mechanism. 
         [0020]    By performing a scan in a state where the cradle  204  is stopped, an axial scan can be performed. By continuously performing a scan a plurality of times while successively moving the cradle  204 , a helical scan can be conducted. By performing a scan in each of stop positions while intermittently moving the cradle  204 , a cluster scan can be carried out. 
         [0021]    The height of the top board  202  is adjusted by making a stay  206  swing around the attachment part to a base  208  around the center. By the swing of the stay  206 , the top board  202  is displaced in the vertical and horizontal directions. The cradle  204  moves in the horizontal direction on the top board  202 , thereby cancelling off the displacement in the horizontal direction of the top board  202 . Depending on scan parameters, a scan is performed in a state where the gantry  100  is tilted. The gantry  100  is tilted by a built-in tilting mechanism. 
         [0022]    The table  200  may be of a type in which the top board  202  is moved vertically to the base  208  as shown in  FIG. 2 . The top board  202  is moved in the vertical direction by a built-in lifting mechanism. In the table  200 , horizontal travel of the top board  202  accompanying the vertical movement does not occur. 
         [0023]      FIG. 3  schematically shows the configuration of an X-ray irradiating/detecting apparatus  110 . The X-ray irradiating/detecting apparatus  110  detects an X-ray  134  emitted from a focal point  132  of an X-ray tube  130  by an X-ray detector  150 . 
         [0024]    The X-ray  134  is formed as a cone-beam or fan-beam X-ray by a not-shown collimator. The X-ray detector  150  has an X-ray entrance plane  152  which extends two-dimensionally in correspondence with the spread of an X-ray. The X-ray entrance plane  152  is curved so as to construct a part of a cylinder. The center axis of the cylinder passes through the focal point  132 . 
         [0025]    The X-ray irradiating/detecting apparatus  110  rotates around the center axis passing the center of imaging, that is, isocenter O. The center axis is parallel with the center axis of the cylinder a part of which is formed by the X-ray detector  150 . 
         [0026]    The direction of the center axis of rotation is set as the z direction, the direction extending from the isocenter O to the focal point  132  is set as the y direction, and the direction perpendicular to the z direction and the y direction is set as the x direction. The x, y, and z axes are three axes of a rotation coordinate system using the z axis as a center. 
         [0027]      FIG. 4  is a schematic plan view of the x-ray entrance plane  152  of the X-ray detector  150 . In the X-ray entrance plane  152 , detection cells  154  are arranged two-dimensionally in the x and z directions. That is, the X-ray entrance plane  152  is a two-dimensional array of the detection cells  154 . In the case of using a fan-beam X-ray, the X-ray entrance plane  152  may be a one-dimensional array of the detection cells  154 . 
         [0028]    Each of the detection cells  154  serves as a detection channel of the X-ray detector  150 . Accordingly, the X-ray detector  150  takes the form of a multi-channel X-ray detector. The detection cell  154  is formed by, for example, a combination of a scintillator and a photo diode. 
         [0029]    The contrast imaging of the apparatus will be described.  FIG. 5  is a flowchart of the contrast imaging. The contrast imaging is performed under control of the operator console  300 . As shown in  FIG. 5 , an imaging range is set in step  501 . The imaging range is set by the operator with the operator console  300 . As a result, for example, an imaging range L as shown in  FIG. 6  is set. The imaging range L is a range from the position A to the position B on the body axis of the subject  10 . 
         [0030]    In step  503 , a contrast agent monitoring position is set. The contrast agent monitoring position is set by the operator with the operator console  300  or may be automatically set in association with the setting of the imaging range L. For example, contrast agent monitoring positions a, b, c, and d as shown in  FIG. 6  are set. Although the case where the number of contrast agent monitoring positions is four is described here, the number of contrast agent monitoring positions to be set is not limited to four but may be proper number. Hereinbelow, the contrast agent monitoring position will be also simply called a monitoring position. 
         [0031]    The monitoring position “a” is the uppermost position of a passage of the contrast agent, and the monitoring positions “b”, “c”, and “d” are sequentially on the downstream of the monitoring position “a”. The monitoring position “a” is set out of the imaging range, and the monitoring positions “b”, “c”, and “d” are set in the imaging range. The monitoring position “a” may be set in the head position A of the imaging range. The monitoring position “a” corresponds to a first monitoring position of the invention, and the monitoring positions “b”, “c”, and “d” correspond to second monitoring positions of the invention. 
         [0032]    In step  505 , a base scan is performed. The base scan is performed at each of the monitoring positions “a”, “b”, “c”, and “d” in a state where the contrast agent is not injected to the subject  10 , thereby obtaining slice images  12   a ,  12   b ,  12   c , and  12   d  at the monitoring positions “a”, “b”, “c”, and “d”, respectively. 
         [0033]    In step  507 , an ROI is set. The ROI setting is made by the operator using the operator console  300 . For example, regions of interest (ROI)  14   a ,  14   b ,  14   c , and  14   d  are set for the slice images  12   a ,  12   b ,  12   c , and  12   d , respectively. The regions of interest (ROI) are regions in the body through which the contrast agent is expected to pass. Although one region of interest is set for each of the slice images, a plurality of regions of interest may be set for each slice image. The number of regions of interest may vary among slice images. 
         [0034]    In step  509 , a monitoring scan is performed. The monitoring scan is a scan performed with a dose lower than that of the main scan which will be described later, in order to monitor arrival of the contrast agent. The monitoring scan starts concurrently with injection of the contrast agent into the subject  10 . The monitoring scan is performed, first, at the monitoring position “a”, thereby obtaining the slice image  12   a.    
         [0035]    In step  511 , whether a CT number exceeds a threshold or not is determined by the operator console  300 . The CT number is of the ROI  14   a . The threshold is determined so as to be larger than the CT number when the contrast agent does not flow in the ROI  14   a  and smaller than the CT number when the contrast agent flows in the ROI  14   a.    
         [0036]    When the CT number does not exceed the threshold, the program returns to step  509  and the monitoring scan is continued. The slice image  12   a  is updated and the CT number of the updated slice image  12   a  undergoes the determination in step  511 . For the time the CT number does not exceed the threshold, the operations in steps  509  and  511  are repeated. 
         [0037]    When the contrast agent reaches the monitoring position “a” and the CT number in the ROI  14   a  exceeds the threshold, it is determined in step  511  that the CT number exceeds the threshold. That is, arrival of the contrast agent at the monitoring position “a” is detected. By detecting the arrival of the contrast agent, the program moves to step  513 . 
         [0038]    In step  513 , the main scan is started. The main scan is a scan for acquiring a slice image actually used for diagnosis or the like. A helical scan starts from the head of the imaging range L. In parallel with the scan, image reconstruction is also performed. By the helical scan, the scan whose scan position changes from one end A to the other end B of the imaging range L is performed. The scan position changes continuously. The first control unit  311  controls from the start of the monitoring scan to the start of the main scan. 
         [0039]    In place of the helical scan, the cluster scan may be conducted. In the case of the cluster scan, the scan position changes step by step. Although the example using the helical scan will be described below, the description is similar to that of the cluster scan. 
         [0040]    In step  515 , whether there is the next monitoring position or not is determined. Since the next monitoring position “b” is set, the program moves to step  517 . In step  517 , immediately after completion of the scan in the monitoring position “b”, image reconstruction is performed. That is, prior to the other images which are reconstructed in parallel until then, the slice image  12   b  is reconstructed. As a result, the slice image  12   b  in the monitoring position “b” can be obtained without delay. 
         [0041]    In step  519 , whether the CT number exceeds the threshold or not is determined. The determination is made with respect to the ROI  14   b  of the slice image  12   b . When a plurality of ROI  14   b  are set, whether any of the CT numbers exceeds the threshold or not is determined. Alternatively, whether an average value of the CT numbers of a plurality of ROI exceeds the threshold or not may be determined. In the following description, it will be applied similarly. 
         [0042]    When the CT number of the ROI  14   b  does not exceed the threshold, the program returns to the monitoring scan of step  509 . The monitoring scan in step  509  is performed in the monitoring position “b”. The slice image  12   b  is updated and, with respect to the image, the CT number in the ROI  14   b  exceeds the threshold or not is determined in step  511 . 
         [0043]    For the time the CT number does not exceed the threshold, the monitoring scan is continued and, with respect to each of slice images obtained, the CT number in the ROI  14   b  is determined on the basis of the threshold. When the CT number does not exceed the threshold, it shows that the contrast agent has not reached the monitoring position. In such a case, the program waits for the arrival of the contrast agent while performing the monitoring scan in the monitoring position “b”. 
         [0044]    When the contrast agent reaches the monitoring position “b” and the CT number in the ROI  14   b  exceeds the threshold, based on the determination in step  511 , the program moves to step  513  and starts the scan. In such a manner, the helical scan restarts from the monitoring position “b”. 
         [0045]    On the other hand, in the case where it is determined in step  519  that the CT number in the ROI  14   b  exceeds the threshold, the program returns to step  515  and determines whether there is the next monitoring position or not. Since the next monitoring position “c” is set, in step  517 , the slice image  12   c  in the monitoring position “c” is reconstructed. In step  519 , whether the CT number of the ROI  14   c  exceeds the threshold or not is determined. 
         [0046]    In the following, with respect to the monitoring positions “c” and “d”, operations similar to those for the monitoring position “b” are performed. When there is not next monitoring position, in step  521 , the scan is performed to the end point of the imaging range L, and completes. As described above, whether the contrast agent has reached a monitoring position or not is determined during a scan. When the contrast agent has not reached a monitoring position, the program waits for arrival of the contrast agent while performing the monitoring scan. After confirming that the contrast agent has reached a monitoring position, the helical scan is continued. Consequently, the contrast imaging can be properly performed in the whole imaging range L. Monitoring in the monitoring positions “b”, “c”, and “d” and control of a scan according to the result of monitoring is performed by the second control unit  312 . 
         [0047]    Successive detection of the contrast agent during a scan may be automatically performed in many positions in the imaging range without presetting the monitoring positions. When the monitoring agent has not reached a monitoring position, the monitoring scan may be performed in the position.