Abstract:
An X-ray CT apparatus comprises three X-ray generating units, three groups of detecting elements and a reconstructing unit. The three X-ray generating units are arranged so as to make an angle formed by a first X-ray exposure direction and a second X-ray exposure direction on a rotational plane thereof be a same angle as that formed by the second X-ray exposure direction and a third X-ray exposure direction on the rotational plane, the same angle being smaller than 120 degree. The three groups of detecting elements are arranged opposite to the three X-ray generating units respectively so as to make a field of view formed in a center wider than two side fields of view. The reconstructing unit is configured to reconstruct an image using detection data detected by at least desired one of the three groups of the detecting elements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an X-ray CT (computed tomography) apparatus and a data detecting method of X-ray CT apparatus reconstructing a diagnostic image of an object using X-ray detection data acquired by exposing an X-ray to the object, and more particularly, to an X-ray CT apparatus and a data detecting method of X-ray CT apparatus having a plurality of tubes. 
     2. Description of the Related Art 
     An X-ray CT apparatus reconstructs a diagnostic image of an object by applying an X-ray from an X-ray tube to the object, acquiring X-ray detection data by an X-ray detector, and subjecting the acquired data to image reconstruction processing. Half reconstruction is one method for reconstructing a diagnostic image with the X-ray CT apparatus. In contrast to normal image reconstruction processing that reconstructs a diagnostic image by detecting data on an object over a range of 360°, half reconstruction reconstructs a diagnostic image by generating one image on the basis of projection data for an angle less than 360° (in general, projection data for the sum of 180° and the fan angle). 
     Half reconstruction is frequently used to obtain an image of a part, such as the heart, which moves locally, because of its capabilities of reconstructing an image from projection data for a narrow angle range and obtaining an image with high time resolution. In particular, when an image of the heart is obtained by half reconstruction using X-ray detection data for an angle less than 360° acquired in synchronization with an electrocardiogram (ECG), the influence of the heartbeats on the image can be reduced. 
       FIG. 10  is a conceptual diagram explaining the method for acquiring data for half reconstruction in synchronization with an electrocardiogram by the conventional X-ray CT apparatus. 
     As shown in  FIG. 10 , an X-ray is applied from a tube # 1  to an object, and half data Dh is acquired by an X-ray detector during the heartbeats shown in an ECG signal. The angle range of the half data Dh in  FIG. 10  is set in the range of approximately 210° to 240°, for example, at 240°. 
     When the number of heartbeats per minute is 120 bpm, the length tb of one heartbeat is 0.5 sec, and therefore, high corresponding time resolution is required in order to acquire data within one heart beat. In normal cases, however, it is often difficult to sufficiently acquire half data within one heartbeat. Accordingly, a method of acquiring half data in divided segments within a plurality of heartbeats has been proposed. 
       FIG. 11  is a conceptual diagram explaining the method for acquiring data for half reconstruction from divided segments in synchronization with an electrocardiogram by the conventional X-ray CT apparatus. 
     In the proposed method, while a tube is helically moved around an object, as shown by a tube locus p in  FIG. 11 , a first segment S 1  of half data is acquired within a first heartbeat B 1 , and second and third segments S 2  and S 3  of the half data are acquired within second and third heartbeats B 2  and B 3  respectively. By data acquisition within three heartbeats, half data for approximately 240° on a reconstruction plane Y can be obtained. 
     On the other hand, a multi-tube X-ray CT apparatus have been proposed as an attempt to increase the time resolution. In the multi-tube X-ray CT apparatus, X-rays are emitted from a plurality of tubes to an object, and are detected by X-ray detectors arranged opposite to the corresponding tubes. 
     One of the multi-tube X-ray CT apparatuses is a three-tube X-ray CT apparatus including three tubes. In the proposed three-tube X-ray CT apparatus, three pairs of a tube and a detector are equally spaced 120° apart (see, for example, Japanese Patent Application (Laid-Open) No. H5-168616 or Japanese Patent Application (Laid-Open) No. 2001-346791). The three-tube X-ray CT apparatus enables 360° data to be acquired by rotating each pair 120°. For this reason, ideally, it is expected to acquire data in one third of the time taken for an X-ray CT apparatus including only one tube to acquire data. 
       FIG. 12  is a conceptual diagram explaining the method for acquiring data by the conventional proposed three-tube X-ray CT apparatus. 
     When three tubes # 1 , # 2 , and # 3  are equally spaced 120° apart in a state St 01  in  FIG. 12 , 360° data can be acquired by rotating the tubes # 1 , # 2 , and # 3  120° into a state St 02 . 
     Therefore, when rotation speed of the tubes is 0.3 sec/rot, the time needed to acquire 360° data is 120/360×0.3=0.1 (sec), which is one third of the time taken when one tube is used. 
       FIG. 13  is a conceptual diagram showing data acquired by the conventional proposed three-tube X-ray CT apparatus. 
     In  FIG. 13 , the ordinate indicates the data acquisition range expressed as the angle of application of X-rays to an object, and the abscissa indicates the used channels (Ch) of the X-ray detectors. As shown in  FIG. 13 , X-ray detectors # 1 ′, # 2 ′, and # 3 ′ opposing the three corresponding tubes # 1 , # 2 , and # 3  acquire different data for each 120°. The X-ray detectors # 1 ′, # 2 ′, and # 3 ′ are equivalent in terms of the number of channels of detecting elements provided therein, and X-rays are detected in all the channels. 
     As a result, the equivalent number of data of 120° data D# 1 ′, D# 2 ′ and D# 3 ′ according to the number of the channels is acquired by the X-ray detectors # 11 , # 21  and # 3 ′ respectively, as shown in  FIG. 13 . That is, 360° data that is proportional to the number of the channels is acquired by the three-tube X-ray CT apparatus. 
     Such a multi-tube X-ray CT apparatus is considered effective for high-speed scanning from a viewpoint of time resolution. 
     In the half reconstruction technology, enhancement of the time resolution is important, as described above. However, when half reconstruction is performed by a conventional one-tube X-ray CT apparatus, the time resolution is insufficient, and as a result, it is sometimes difficult to acquire necessary data within one heartbeat. For this reason, data is acquired in segments over a plurality of heartbeats. 
     Accordingly, the use of a multi-tube X-ray CT apparatus for such data acquisition that requires high time resolution is expected. However, when an image of a local part, such as the heart, is obtained with a multi-tube X-ray CT apparatus including equally spaced tubes, half reconstruction is not used, but full reconstruction is performed to acquire angularly continuous data on imaging. 
     In the multi-tube X-ray CT apparatus including equally spaced tubes, the tubes need not be rotated 360° because of the number of tubes, and this makes the time resolution higher than when full reconstruction is performed with a one-tube X-ray CT apparatus. On the other hand, it is difficult to use half reconstruction. For this reason, there is a demand to further increase the time resolution on imaging of a local part. 
     Further, when a plurality of pairs of a tube and an X-ray detector are provided to ensure a sufficient FOV (field of view) on the multi-tube X-ray CT apparatus, the manufacturing cost increases. In general, high time resolution is particularly required mainly in a case in which the FOV is narrow and an image of a local part, such as the heart, is obtained. Therefore, the time resolution is also expected to be increased with a simpler configuration. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in light of the conventional situations, and it is an object of the present invention to provide an X-ray CT apparatus and a data detecting method of X-ray CT apparatus which can image with time resolution and FOV according to a purpose by exposing X-rays from a plurality of tubes to an object. 
     Furthermore, it is another object of the present invention to provide an X-ray CT apparatus and a data detecting method of X-ray CT apparatus which can image with locally improved time resolution on half reconstruction by exposing X-rays from a plurality of tubes to an object. 
     The present invention provides an X-ray CT apparatus comprising: three X-ray generating units arranged so as to make an angle formed by a first X-ray exposure direction and a second X-ray exposure direction on a rotational plane thereof be a same angle as that formed by the second X-ray exposure direction and a third X-ray exposure direction on the rotational plane, the same angle being smaller than 120 degree; three groups of detecting elements arranged opposite to the three X-ray generating units respectively so as to make a field of view formed in a center wider than two side fields of view; and a reconstructing unit configured to reconstruct an image using detection data detected by at least desired one of the three groups of the detecting elements, in an aspect to achieve the object. 
     The present invention also provides an X-ray CT apparatus comprising: X-ray generating units arranged so as to expose X-rays in mutually different rotational radius directions and make angles formed by adjacent two X-ray exposure directions unequal mutually; groups of detecting elements arranged opposite to the X-ray generating units respectively; a supporting member supporting at least two of the groups in common; and a reconstructing unit configured to reconstruct an image using detection data from at least desired one of the groups, in an aspect to achieve the object. 
     The present invention also provides an X-ray CT apparatus comprising: a first X-ray generating unit configured to expose an X-ray in a first exposure direction; a second X-ray generating unit configured to expose an X-ray in a second exposure direction which is different from the first exposure direction on a rotational plane thereof; a first group of detecting elements of which at least one of a size and a pitch between adjacent detecting elements of some detecting elements is different from that of other detecting elements, the first group being opposite to the first X-ray generating unit; a second group of detecting elements opposite to the second X-ray generating unit; and a reconstructing unit configured to reconstruct an image using data detected by at least one of the first group and the second group, in an aspect to achieve the object. 
     The present invention also provides a data detecting method of X-ray CT apparatus comprising steps of: exposing an X-ray from at least one of three X-ray generating units arranged in positions so as to make an angle formed by a first X-ray exposure direction and a second X-ray exposure direction on a rotational plane thereof be a same angle as that formed by the second X-ray exposure direction and a third X-ray exposure direction on the rotational plane, the same angle being smaller than 120 degree; detecting an exposed X-ray as detection data using at least one of three groups of detecting elements arranged opposite to the three X-ray generating units respectively so as to make a field of view formed in a center wider than two side fields of view; and reconstructing an image using the detection data, in an aspect to achieve the object. 
     The present invention also provides a data detecting method of X-ray CT apparatus comprising steps of: exposing an X-ray from at least one of X-ray generating units arranged so as to expose X-rays in mutually different rotational radius directions and make angles formed by adjacent two X-ray exposure directions unequal mutually; detecting an exposed X-ray as detection data by at least one of groups of detecting elements arranged opposite to the X-ray generating units respectively, at least two of the groups being supported by a common supporting member; and reconstructing an image using the detection data, in an aspect to achieve the object. 
     The present invention also provides a data detecting method of X-ray CT apparatus comprising steps of: exposing an X-ray from at least one of a first X-ray generating unit for exposing an X-ray in a first exposure direction and a second X-ray generating unit for exposing an X-ray in a second exposure direction which is different from the first exposure direction on a rotational plane thereof; detecting an exposed X-ray as detection data by at least one of a first group of detecting elements of which at least one of a size and a pitch between adjacent detecting elements of some detecting elements is different from that of other detecting elements and a second group of detecting elements opposite to the second X-ray generating unit, the first group being opposite to the first X-ray generating unit; and reconstructing an image using the detection data, in an aspect to achieve the object. 
     The X-ray CT apparatus and the data detecting method of X-ray CT apparatus as described above make it possible to image with time resolution and FOV according to a purpose by exposing X-rays from a plurality of tubes to an object. 
     Furthermore, it is possible to image with locally improved time resolution on half reconstruction by exposing X-rays from a plurality of tubes to an object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a functional block diagram showing an X-ray CT apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a diagram showing the changes of the positions of the X-ray detectors in the X-ray CT apparatus shown in  FIG. 1  during detection of data for half reconstruction using the three X-ray detectors; 
         FIG. 3  is a conceptual diagram explaining the method for detecting data for half reconstruction using the three X-ray detectors in synchronization with an ECG signal by the X-ray CT apparatus shown in  FIG. 1 ; 
         FIG. 4  is another conceptual diagram explaining the method for detecting data for half reconstruction using the three X-ray detectors in synchronization with an ECG signal by the X-ray CT apparatus shown in  FIG. 1 ; 
         FIG. 5  is a conceptual diagram showing data for half reconstruction acquired by the three X-ray detectors of the X-ray CT apparatus shown in  FIG. 1 ; 
         FIG. 6  is a functional block diagram showing a modified example of the X-ray CT apparatus shown in  FIG. 1 ; 
         FIG. 7  is a diagram showing a structure of an X-ray detector included in an X-ray CT apparatus according to a second embodiment of the present invention; 
         FIG. 8  is a diagram explaining the method for detecting data in case of acquiring the data from the wide FOV W  using the X-ray detector shown in  FIG. 7 ; 
         FIG. 9  is a diagram explaining the method for detecting data in case of acquiring the data from the local FOV L  using the X-ray detector shown in  FIG. 7 ; 
         FIG. 10  is a conceptual diagram explaining the method for acquiring data for half reconstruction in synchronization with an electrocardiogram by the conventional X-ray CT apparatus; 
         FIG. 11  is a conceptual diagram explaining the method for acquiring data for half reconstruction from divided segments in synchronization with an electrocardiogram by the conventional X-ray CT apparatus; 
         FIG. 12  is a conceptual diagram explaining the method for acquiring data by the conventional proposed three-tube X-ray CT apparatus; and 
         FIG. 13  is a conceptual diagram showing data acquired by the conventional proposed three-tube X-ray CT apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An X-ray CT apparatus and a data detecting method of X-ray CT apparatus according to embodiments of the present invention will be described with reference to the accompanying drawings. 
       FIG. 1  is a functional block diagram showing an X-ray CT apparatus according to a first embodiment of the present invention. 
     An X-ray CT apparatus  1  includes a gantry  2 , a computer  3 , and an ECG unit  4 . The gantry  2  includes a high-voltage generator  5 , a drive control unit  6 , a plurality of X-ray tubes, such as three tubes  7   a ,  7   b , and  7   c , for emitting X-rays from focus portions (X-ray emitting portions), X-ray detectors  9   a ,  9   b , and  9   c  arranged opposed to the corresponding tubes  7   a ,  7   b , and  7   c  to form pairs  8   a ,  8   b , and  8   c , and a data acquisition system (DAS)  10 . 
     The tubes  7   a ,  7   b , and  7   c  and the X-ray detectors  9   a ,  9   b , and  9   c  are arranged on a common rotating member (not shown), and are rotated on the same plane by rotating the rotating member. The rotating member is rotatably supported on a gantry fixing unit by a bearing. 
     The high-voltage generator  5  provided in the gantry  2  applies a high-voltage to k-ray tubes constituted by the tubes  7   a ,  7   b , and  7   c  so that the tubes  7   a ,  7   b , and  7   c  emit X-rays to an object. The drive control unit  6  rotates the rotating member to rotate the tubes  7   a ,  7   b , and  7   c  and the X-ray detectors  9   a ,  9   b , and  9   c  opposed to the tubes. 
     That is, in the X-ray CT apparatus  1 , the pairs  8   a ,  8   b , and  8   c  respectively formed by the tubes  7   a ,  7   b , and  7   c  and the X-ray detectors  9   a ,  9   b , and  9   c  are rotated by the driving control unit  6 , and a high voltage is applied from the high-voltage generator  5  to the tubes  7   a ,  7   b , and  7   c . By the application of the high voltage, X-rays are emitted from the tubes  7   a ,  7   b , and  7   c  to an object, and are then detected by the X-ray detectors  9   a ,  9   b , and  9   c  opposed to the corresponding tubes  7   a ,  7   b , and  7   c . X-ray detected data obtained by the X-ray detectors  9   a ,  9   b , and  9   c  are given to the DAS  10  so as to be converted into digital raw data. 
     The pairs  8   a ,  8   b , and  8   c  of the tubes  7   a ,  7   b , and  7   c  and the X-ray detectors  9   a ,  9   b , and  9   c  are unequally spaced so that data acquisition can be properly performed by image reconstruction, preferably, half reconstruction, so that the pairs  8   a ,  8   b , and  8   c  cover at least the required ranges, and so that one, two, or all of the angles formed between the adjoining pairs  8   a ,  8   b , and  8   c  in the direction of application of the X-rays are different. Preferably, the pairs  8   a ,  8   b , and  8   c  are arranged at the positions corresponding to the angles obtained by equally dividing the angle range necessary for data acquisition by half reconstruction. That is, it is preferable that the tubes  7   a ,  7   b , and  7   c  be arranged at the positions corresponding to the angles obtained by equally dividing an appropriate angle range for half reconstruction and that the X-ray detectors  9   a ,  9   b , and  9   c  be arranged opposed to the corresponding tubes  7   a ,  7   b , and  7   c  respectively. 
     For example, when the appropriate angle for data acquisition by half reconstruction is 240° and three tubes  7  are provided, as shown in  FIG. 1 , the other pairs  8   a  and  8   c  are arranged on both sides of the pair  8   b  serving as the reference so as to cross the pair  8   b  at α 1 =α 2 =80° which is one third of 240°. The reference pair is not limited to the center pair  8   b , and may be any of the pairs  8   a ,  8   b , and  8   c . Further, the angles α 1  and α 2  shown in  FIG. 1  may be different from each other. 
     While it is, in actuality, possible to acquire data for a range enlarged by the fan beam angle of the X-ray, herein, consideration is given only to the positional relationship among the center lines of the pairs of  8   a ,  8   b , and  8   c  for simple explanation. 
     The X-ray detectors  9   a ,  9   b , and  9   c  are two-dimensional detectors, and each include a plurality of rows of detecting elements for a plurality of channels arranged along the direction of the body axis (direction perpendicular to the plane of  FIG. 1 ). While each of the X-ray detectors  9   a ,  9   b , and  9   c  includes a plurality of detecting elements, the X-ray detector  9  in at least one pair  8  includes a number of detecting elements corresponding to a sufficient number of channels to cover a wide FOV needed to obtain an image of the entire cross section of the object by full reconstruction or half reconstruction. On the other hand, the X-ray detector  9  in the other pair  8  includes a number of detecting elements corresponding to a sufficient number of channels to cover an appropriate local FOV (narrower than the above-described wide FOV) for imaging by half reconstruction. In this embodiment, the local FOV has a size that is proper for imaging of the heart. 
     For example, as shown in  FIG. 1 , the X-ray detector  9   b  of the center pair  8   b  includes a number of detecting elements corresponding to approximately 1000 channels that sufficiently cover a wide FOV having a diameter D 1  of approximately 500 mm suited for imaging by full reconstruction or half reconstruction. Each of the X-ray detectors  9   a  and  9   b  in the side pairs  8   a  and  8   c  includes a number of detecting elements corresponding to approximately 500 channels that sufficiently cover a local FOV having a diameter D 2  of approximately 200 mm suited for imaging by half reconstruction. 
     The computer  3  includes an input device  11 , a display unit  12 , a scan control unit  13 , a full image reconstructing unit  14  as an example of a second image reconstructing unit, a half image reconstructing unit  15  as an example of a first image reconstructing unit, a reconstructed image storage unit  16 , an absorption correction unit  17 , and a display processing unit  18 . All or some of the above-described elements can be constructed with circuits or by reading a data processing program into an operation device (not shown). 
     The ECG unit  4  obtains an ECG signal of the object, and sends the ECG signal to the scan control unit  13 . 
     The scan control unit  13  is triggered by the ECG signal received from the ECG unit  4 , and outputs a control signal to the high-voltage generator  5  to execute ECG-synchronized scanning. Further, the scan control unit  13  determines the imaging range and whether imaging is to be performed by full reconstruction or half reconstruction in accordance with an instruction inputted from the input device  11 . Depending on the determined reconstruction method, the scan control unit  13  sends control signals to the high-voltage generator  5  and the drive control unit  6  so that it can control which of the tubes  7   a ,  7   b , and  7   c  emit X-rays, the emission timing, and the rotation angle of the tubes  7   a ,  7   b , and  7   c  and the X-ray detectors  9   a ,  9   b , and  9   c.    
     Under the control of the scan control unit  13 , data acquisition is performed with the X-ray detector  9   b  including the detecting elements that cover a wide FOV on imaging for a wide range, and with a plurality of X-ray detectors  9  including the detecting elements which cover a local FOV on imaging for a local range by half reconstruction, preferably, all the X-ray detectors  9   a ,  9   b , and  9   c.    
     In other words, the scan control unit  13  switches between imaging modes on the basis of a command from the input device  11 . It is possible to set, as the imaging modes, a first imaging mode in which imaging is performed by full reconstruction over a wide FOV only with the center large X-ray detector  9   b , as described above, and a second imaging mode in which imaging is performed by half reconstruction over a local FOV with all the X-ray detectors  9   a ,  9   b , and  9   c.    
     The first imaging mode can cover a wide FOV. While the FOV is local in the second imaging mode, the number of detecting elements used for data acquisition is large in the rotating direction of the X-ray detectors  9   a ,  9   b , and  9   c , and therefore, the time necessary for data acquisition can be made shorter than in the first imaging mode. This can achieve a high time resolution for a local FOV. For this reason, the second imaging mode is suitable for, for example, scanning the heart. An imaging mode may be set in which reconstruction other than half reconstruction is performed for data acquired from a local FOV. 
     Further, an arbitrary imaging condition can be set as an imaging mode. Other imaging modes that are effective when data acquisition is performed with the three X-ray detectors  9   a ,  9   b , and  9   c , as shown in  FIG. 1 , are, for example, a third imaging mode in which the two X-ray detectors  9   a  and  9   b , that is, the large X-ray detector  9   b  covering a wide FOV and one small X-ray detector  9   a  covering a local FOV are used and in which different energies (tube voltages) are applied to the X-ray detectors  9   a  and  9   b , and a fourth imaging mode in which two small X-ray detectors  9   a  and  9   c  covering a local FOV are used and in which different energies (tube voltages) are applied to the X-ray detectors  9   a  and  9   c.    
     In the third imaging mode, two images having different contrasts can be obtained by using data from a wide FOV and data from a local FOV. By combining the two images, an image having contrast that is diagnostically useful can be obtained. In the fourth imaging mode, the crossing angle formed by the paths of X-rays exposed from the two tubes  7   a  and  7   c  is larger than in the third imaging mode. For this reason, the fourth imaging mode can reduce the occurrence and influence of scattered rays. 
     In the third and fourth imaging modes, control information about the voltage to be generated and information about which of the tubes  7   a ,  7   b , and  7   c  is to be used is sent from the scan control unit  13  to the high-voltage generator  5  in order to adjust the tube voltages to be applied to the X-ray detectors  9   a  and  9   b  or the X-ray detectors  9   a  and  9   c.    
     The first and second imaging modes will be described below. 
     The full image reconstructing unit  14  obtains, from the DAS  10 , raw data acquired from an FOV for full reconstruction, and reconstructs image data by subjecting the raw data to image reconstructing processing. The full image reconstructing unit  14  also writes the reconstructed image data in the reconstructed image storage unit  16  to be stored in it. That is, the full image reconstructing unit  14  reconstructs image data by full reconstruction using data detected by the detecting elements of the X-ray detector  9   b  that can acquire data from a FOV for full reconstruction wider than a local FOV for half reconstruction. 
     The half image reconstructing unit  15  obtains, from the DAS  10 , raw data acquired from a local region for half reconstruction, and reconstructs local image data by subjecting the raw data to image reconstructing processing for half reconstruction. The half image reconstructing unit  15  also writes the reconstructed image data in the reconstructed image storage unit  16  to be stored in it. 
     The absorption correction unit  17  subjects the local image data, which is reconstructed by half reconstruction and is stored in the reconstructed image storage unit  16 , to absorption correction using data acquired from the outside of the local region. That is, the absorption correction unit  17  makes absorption correction to the image data reconstructed by the half image reconstructing unit  15  by using data from outside the local region, of the data detected by the detecting elements of the X-ray detector  9   b  that can acquire data from the region wider than the local region. 
     The display processing unit  18  generates image signals by subjecting the image data stored in the reconstructed image storage unit  16  to display processing, and sends the generated image signals to the display unit  12  to display images. 
     The action and operation of the X-ray CT apparatus  1  will now be described. A description will be given of a case in which three pairs  8  are provided and in which two pairs  8   a  and  8   c  each covering a local FOV are arranged on either side of a center pair  8   b  covering a wide FOV so as to cross the center pair  8   b  at an angle of 80°, as shown in  FIG. 1 . 
     First, the input device  11  directs the scan control unit  13  to perform a wide-range imaging of an object by full reconstruction or local imaging of, for example, the heart by half reconstruction. For example, when the scan control unit  13  is directed to perform wide-range imaging of the object by full reconstruction, the center pair  8   b  is used for imaging. 
     That is, in synchronization with an ECG signal from the ECT unit  4 , control signals are given from the scan control unit  13  to the high-voltage generator  5  and to the drive control unit  6 . An X-ray is applied from the center tube  7   b  to an object (not shown), passes through the object, and is detected by the X-ray detector  9   b  that covers a wide FOV. The detected X-rays are converted into raw data by the DAS  10 , which is then output to the full image reconstructing unit  14 . 
     The full image reconstructing unit  14  generates image data by executing image reconstruction processing using only the raw data obtained via the X-ray detector  9   b  that covers the wide FOV. Therefore, wide-range image data is generated by the full image reconstructing unit  14 . The generated image data is appropriately stored in the reconstructed image storage unit  16 , and is then given as image signals from the display processing unit  18  to the display unit  12  so as to enable the image to be displayed. 
     For example, when the scan control unit  13  is directed to perform local imaging of the heart by half reconstruction over a data acquisition range of 240°, all the three pairs  8   a ,  8   b , and  8   c  are used for imaging. 
     That is, X-rays are exposed from all the three tubes  7   a ,  7   b , and  7   c  onto an object (not shown) in synchronization with an ECG signal from the ECG unit  4 , pass through the object, and are detected by the center X-ray detector  9   b  covering the wide FOV and the two side X-ray detectors  9   a  and  9   c  covering the local FOV. In this case, while data acquisition ranges of the X-ray detectors  9   a ,  9   b , and  9   c  may slightly overlap, they are different in substance. 
       FIG. 2  is a diagram showing the changes of the positions of the X-ray detectors  9   a ,  9   b , and  9   c  in the X-ray CT apparatus  1  shown in  FIG. 1  during detection of data for half reconstruction using the three X-ray detectors  9   a ,  9   b , and  9   c.    
     Since the data acquisition ranges of the three X-ray detectors  9   a ,  9   b , and  9   c  are different, when the total data acquisition range is 240°, necessary data can be acquired by rotating the X-ray detectors  9   a ,  9   b , and  9   c  80° from a state St 1  to a St 2 , as shown in  FIG. 2 . That is, half image reconstruction can be performed by the multi-tube X-ray CT apparatus  1 . 
       FIG. 3  is a conceptual diagram explaining the method for detecting data for half reconstruction using the three X-ray detectors  9   a ,  9   b , and  9   c  in synchronization with an ECG signal by the X-ray CT apparatus  1  shown in  FIG. 1 .  FIG. 4  is another conceptual diagram explaining the method for detecting data for half reconstruction using the three X-ray detectors  9   a ,  9   b , and  9   c  in synchronization with an ECG signal by the X-ray CT apparatus  1  shown in  FIG. 1 . 
     As shown in  FIG. 3 , data for half reconstruction are simultaneously detected by the three X-ray detectors  9   a ,  9   b , and  9   c  during the heartbeats shown in an ECG signal. That is, X-rays applied from the three tubes  7   a ,  7   b , and  7   c  are simultaneously detected by the corresponding first (# 1 ), second (# 2 ), and third (# 3 ) X-ray detectors  9   a ,  9   b , and  9   c , thereby acquiring half data Dh. 
     When the number of heartbeats per minute is 120 bpm, the length tb of one heartbeat is 0.5 sec. The time needed to rotate the tubes  7   a ,  7   b , and  7   c  by 80° is 80/360×0.3≈0.07 (sec) when the rotation speed of the tubes  7   a ,  7   b , and  7   c  is 0.3 sec/rot. That is, the time needed to perform data acquisition for 240° is 0.07 sec, and high time resolution with respect to the length of one heartbeat can be achieved. 
     For this reason, as shown by a tube locus p in  FIG. 4 , the tubes  7   a ,  7   b , and  7   c  are helically moved around the object, and data for 240° in a reconstruction plane Y are simultaneously acquired from separate regions by the first (# 1 ), second (# 2 ), and third (# 3 ) X-ray detectors  9   a ,  9   b , and  9   c  within one heartbeat. Further, data acquisition can sometimes be performed a plurality of times during one heartbeat, as shown in  FIG. 3 . For example, data accuracy can be increased by acquiring data a plurality of times and averaging the data. 
     That is, as shown by a lower chart in  FIG. 3 , data for a 80° segment, of the 240° for half reconstruction, is acquired by the first (# 1 ) X-ray detector  9   a , data for another 80° segment is acquired by the second (# 2 ) X-ray detector  9   b , and data for the remaining 80° segment is acquired by the third (# 3 ) X-ray detector  9   c.    
     The X-ray detection data thus acquired by the X-ray detectors  9   a ,  9   b , and  9   c  are converted into raw data by the DAS  10 , are combined for half reconstruction, and are then outputted to the half image reconstructing unit  15 . 
       FIG. 5  is a conceptual diagram showing data for half reconstruction acquired by the three X-ray detectors  9   a ,  9   b , and  9   c  of the X-ray CT apparatus  1  shown in  FIG. 1 . 
     In  FIG. 5 , the abscissa indicates the channel (Ch) to which the acquired data belong, and the ordinate indicates the data acquisition range expressed by the angle of application of the X-rays onto the object. 
     As shown in  FIG. 5 , data D# 1 , D# 2 , and D# 3  for each 80° are respectively acquired by the first (# 1 ), second (# 2 ), and third (# 3 ) X-ray detectors  9   a ,  9   b , and  9   c , and consequently, data for the total angle of 240° that is appropriate for half reconstruction can be obtained. 
     Since the first (# 1 ) and third (# 3 ) X-ray detectors  9   a  and  9   c  each include a number of detecting elements only corresponding to the number of channels that cover the local FOV, they acquire data D# 1  and D# 3  on fewer channels than those in case of the second (# 2 ) X-ray detector  9   b  including a number of detecting elements corresponding to the channels that cover the wide FOV. In other words, data D# 1  and D# 3  acquired by the first (# 1 ) and third (# 3 ) X-ray detectors  9   a  and  9   c  are obtained only from the channels in the local FOV. 
     In contrast, data D# 2  acquired by the second (# 2 ) X-ray detector  9   b  can be divided into data D# 2   a  from channels within the local FOV and data D# 2   b  from channels within the wide FOV outside the local FOV. 
     Therefore, the half image reconstructing unit  15  reconstructs image data by using the data D# 1 , D# 2   a , and D# 3  from the channels within the local FOV obtained by the first (# 1 ), second (# 2 ), and third (# 3 ) X-ray detectors  9   a ,  9   b , and  9   c  in an ECG-synchronized manner. The obtained local image data is appropriately stored in the reconstructed image storage unit  16 . In this case, the data D# 2   b , which is acquired by the second (# 2 ) X-ray detector  9   b  from the channels within the wide FOV outside the local FOV, can be added as additional information to the obtained image data for the purpose of absorption correction which will be described below. 
     As required, the absorption correction unit  17  subjects the local image data, which is reconstructed by half reconstruction by the half image reconstructing unit  15  and stored in the reconstructed image storage unit  16 , to absorption correction using the data D# 2   b , serving as the additional information, obtained from the outside the local region. 
     That is, when the local image data is, for example, image data on the heart, it is expressed as a CT value of the heart. However, it is sometimes clinically important to find a CT value of the heart as a relative value with respect to a CT value of air outside the object. Accordingly, in absorption correction, a CT value of the heart can be found as a relative value with respect to a CT value of air on the basis of the difference between a CT value of a tissue near the heart and a CT value of air when the difference can be obtained. 
     In this case, the data D# 2   b  acquired as the additional information from the outside of the local region can be used for absorption correction. 
     Further, the image data is stored again in the reconstructed image storage unit  16  after absorption correction, and is given as image signals from the display processing unit  18  to the display unit  12  so as to enable the image to be displayed. 
     In the above-described X-ray CT apparatus  1 , an image can be obtained with a time resolution and an FOV, which fit the required purpose, by switching between half reconstruction and full reconstruction. In particular, imaging can be performed with a time resolution locally increased by half reconstruction. More specifically, half reconstruction can be performed with a time resolution of 50 to 60 msec. 
     Especially, arranging the three X-ray detectors  9   a ,  9   b  and  9   c  to line symmetry on the rotational plane so as to be near as much as possible mutually and setting the FOV of the center X-ray detector  9   b  wider than that of the two side X-ray detectors  9   a  and  9   c  as shown in  FIG. 1  make it possible to arrange more detecting elements without interference to improve time resolution as well as facilitate processing of data by symmetry property. In addition, setting the distances between the adjacent X-ray detectors  9   a ,  9   b  and  9   c  shorter gives a wider size of FOV which can acquire data with high time resolution. 
     A modification of the X-ray CT apparatus  1  will now be described. 
       FIG. 6  is a functional block diagram showing a modified example of the X-ray CT apparatus  1  shown in  FIG. 1 . 
     As shown in  FIG. 6 , X-ray detectors do not always need to be physically separate as long as they can detect X-rays exposed from a plurality of different directions. That is, a plurality of or a single common detector support frame  20  may be provided inside a rotating frame  21 , and a plurality of groups of detecting elements can be provided as detector units  22   a ,  22   b , and  22   c  on the detector support frame  20 . The rotating frame  21  is typically cast in metal such as aluminum. For this reason, the detector support frame  20  can also be integral cast in metal, such as aluminum, as a part of the rotating frame  21 . In the modification shown in  FIG. 6 , three detector units  22   a ,  22   b , and  22   c  are provided on the single detector support frame  20 , the center detector unit  22   b  can cover a wide FOV, and the two side detector units  22   a  and  22   c  can cover a local FOV, for example. 
     When these detector units  22   a ,  22   b , and  22   c  are used, positional interference among the X-ray detectors  9   a ,  9   b , and  9   c  shown in  FIG. 1  does not occur. For this reason, the distances between the detector units  22   a ,  22   b , and  22   c , that is, between the groups of detecting elements can be reduced. Consequently, a local FOV that allows high time resolution can be enlarged. Moreover, the positioning accuracy of the detecting elements can be increased. In contrast, when a plurality of X-ray detectors  9   a ,  9   b , and  9   c  are separately provided, as shown in  FIG. 1 , production is facilitated. 
     The configuration and structures of the above-described X-ray CT apparatus  1  may be simplified. For example, when a plurality of X-ray detectors  9  are equally spaced, and are made different only in the number of channels of groups of detecting elements, that is, in the size of the FOV which the detectors can cover, the manufacturing cost of the X-ray detectors  9  can be decreased while the time resolution of the multi-tube X-ray CT apparatus can be obtained. Conversely, when X-ray detectors  9  that can cover the same FOV are unequally spaced at appropriate positions for half reconstruction, the manufacturing cost of the X-ray detectors  9  is increased, but it can be expected to provide an apparatus specialized in imaging with high time resolution over a wider FOV and half reconstruction of a local image. 
       FIG. 7  is a diagram showing a structure of an X-ray detector included in an X-ray CT apparatus according to a second embodiment of the present invention. 
     An X-ray CT apparatus  1 A shown in  FIG. 7  is different from the X-ray CT apparatus  1  shown in  FIG. 1  in terms of the sizes and arrangements (center distances) of detecting elements  30  provided in at least one of a plurality of X-ray detectors  9   a ,  9   b , and  9   c , practically, in the X-ray detector  9   b  that covers a wide FOV. Since other structures and operations are substantially the same as those employed in the X-ray CT apparatus  1  shown in  FIG. 1 , only the X-ray detector  9   b  is shown. The same structures are denoted by the same reference numerals, and descriptions thereof are omitted. 
     In the X-ray detector  9   b  of the X-ray CT apparatus  1 A, a plurality of detecting elements  30  are two-dimensionally arrayed in the rotating direction Dr of the X-ray detector  9   b  and in the direction Da of the rotation axis thereof. Some of the detecting elements  30  have a size different from that of the other detecting elements  30 . Further, the center distance (pitch) between some adjoining detecting elements  30  is different from that of the other adjoining detecting elements  30 . At least one of the size and the pitch may be different between the detecting elements. 
     In practice, it is preferable that the pitch Pa between the adjoining detecting elements  30   a  having a small size Xa be short according to the size Xa and that the pitch Pb between the adjoining detecting elements  30   b  having a large size Xb be long according to the size Xb, as shown in  FIG. 7 . In the example shown in  FIG. 7 , the size Xa of some detecting elements  30   a  is half the size Xb of the other detecting elements  30   b , and the pitch Pa between the small detecting elements  30   a  having the half size Xa is half the pitch Pb between the detecting elements  30   b  having the large size Xb. 
     While the sizes Xa and Xb of the detecting elements  30   a  and  30   b  and the pitches Pa and Pb between the detecting elements  30   a  and  30   b  are different in the two-dimensional directions in the X-ray detector  9   b  shown in  FIG. 7 , they may be different only in the rotating direction Dr of the X-ray detector  9   b.    
     When the size of some detecting elements  30  is changed in the X-ray detector  9   b  in this way, sensitivity of the large-sized detecting elements  30  increases to reduce noise, depending on the size X thereof. Conversely, spatial resolution and time resolution of the small-sized detecting elements  30  can be increased depending on the size thereof. That is, sections that are different in sensitivity, time resolution and spatial resolution can be formed on the single X-ray detector  9   b.    
     In contrast, when the pitch P between some detecting elements  30  is changed, noise is reduced in a section of the X-ray detector  9   b  in which the pitch P is long, depending on the pitch P. Moreover, since the structure is simplified, the manufacturing cost can be reduced. Conversely, the time resolution and spatial resolution can be increased in a section of the X-ray detector  9   b  in which the pitch P is short, depending on the pitch P. That is, when the pitch P between some detecting elements  30  is changed, sections that are different in time resolution and spatial resolution can also be formed on the single X-ray detector  9   b.    
     Since a section of the X-ray detector  9   b  for detecting data from the local FOV L  is required to have higher time resolution and higher spatial resolution, detecting elements  30  having a smaller size X are preferably arranged in the section at a shorter pitch P. Conversely, in order to simplify the structure of a section of the X-ray detector  9   b  for detecting data only from a wide FOV w , detecting elements  30  having a size X such as to detect data necessary for at least image reconstruction are preferably arranged in the section at the required pitch P. 
     Accordingly, when the size X of the detecting elements  30  in the two small X-ray detectors  9   a  and  9   c  that cover the local FOV L  shown in  FIG. 1 , and the size Xa of the detecting elements  30   a  in a section for detecting data from the local FOV L  on the large X-ray detector  9   b  that covers the wide FOV W  are made small, and the detecting elements  30  and  30   a  are arranged in the X-ray detectors  9   a ,  9   b , and  9   c  at a small pitch Pa, high time resolution and high spatial resolution with respect to data from the local FOV L  can be obtained. In particular, when the sizes X and Xa and the pitches P and Pa of the detecting elements  30  and  30   a  in the X-ray detectors  9   a ,  9   b , and  9   c  for detecting data from the local FOV L  are set to be equal, data processing can be facilitated. 
     Further, setting the size Xb of the detecting elements  30   b  in the section for detecting data from the outside of the local FOV L  on the large X-ray detector  9   b  that covers the wide FOV W  to one for detecting data necessary for at least image reconstruction and arranging the detecting elements  30   b  at the pitch Pb necessary for at least image reconstruction make it possible to simplify the structure of the X-ray detector  9   b.    
       FIG. 8  is a diagram explaining the method for detecting data in case of acquiring the data from the wide FOV W  using the X-ray detector  9   b  shown in  FIG. 7 .  FIG. 9  is a diagram explaining the method for detecting data in case of acquiring the data from the local FOV L  using the X-ray detector  9   b  shown in  FIG. 7 . 
     When data is acquired from the wide FOV W  with the X-ray detector  9   b , X-rays are detected by both the detecting elements  30   a  having the small size and the small pitch and the detecting elements  30   b  having the large size and the large pitch on the X-ray detector  9   b . Therefore, when charges accumulated in the detecting elements  30   a  and  30   b  are used as detection data without change, the time resolution and spatial resolution are not uniform among the detection data. Accordingly, signal distributing and combining circuits  31  are provided on output sides of the detecting elements  30   a  having the small size, as shown in  FIG. 8 . 
     In case of acquiring data from the wide FOV W , signals outputted from a fixed number of (two in  FIG. 8 ) detecting elements  30   a  are combined to output as single detection data (DATA W 1 , DATA W 2 ) by each of the signal distributing and combining circuits  31 . Consequently, more uniform detection data (DATA W 1 , DATA W 2 , DATA W 3 , DATA W 4 ) can be acquired. 
     In contrast, in case of acquiring data from the local FOV L  with the X-ray detector  9   b , X-rays are detected only by the detecting elements  30   a  having the small size X, as shown in  FIG. 9 . Signals outputted from the detecting elements  30   a  having the small size X are outputted as detection data (DATA L 1 , DATA L 2 , DATA L 3 , DATA L 4 ) by the signal distributing and combining circuits  31 . This can achieve higher spatial resolution and higher time resolution. 
     In the X-ray CT apparatus  1 A shown in  FIG. 7 , the X-ray detectors  9   a ,  9   b , and  9   c  may have the same size, and one or both of the size and pitch of the detecting elements in any of the X-ray detectors  9   a ,  9   b , and  9   c  may be changed in order to increase the time resolution and spatial resolution with respect to data from a local FOV.