preferred embodiments of the present invention will be explained with reference to the attached drawings . fig1 is a block diagram showing the structure of an x - ray ct device according to an embodiment of the present invention , while fig2 is a perspective view of the external structure of the gantry illustrated in fig1 . fig4 is a perspective view of the detector of a two - dimensional array type illustrated in fig1 . the 1pi reconstruction at a high bed moving speed is referred to as “ low radiation exposure mode ”, whereas the 3pi reconstruction at a lower bed moving speed is referred to as “ high image quality mode ”. these modes will be discussed later . at 1pi , an image is reconstructed for a specific slice from projection data equivalent to about 180 degrees . at 3pi , an image is reconstructed for the slice from projection data having a longer collection length ( about 540 degrees ) than 1pi . in an x - ray ct device 10 , a gantry ( also referred to as a base ) 12 that serves as a projection data measurement system contains an x - ray source 14 and a two - dimensional array - type x - ray detector 16 . the x - ray source 14 produces x - ray cone beams having a shape similar to a cone . the two - dimensional array - type x - ray detector 16 is formed of two - dimensionally arranged detecting elements . the x - ray source 14 and the two - dimensional array - type x - ray detector 16 are arranged on a rotation ring 20 to face each other across a subject p positioned on a sliding top of a bed 18 . in the two - dimensional array - type x - ray detector 16 , several one - dimensional array - type detectors , each of which has multiple detecting elements that are one - dimensionally arranged , are stacked together and arranged on the rotation ring 20 . it is assumed that one detecting element corresponds to one channel . the x - rays are applied by the x - ray source 14 onto the subject p by way of an x - ray filter 24 . the x - rays that have passed through the subject p are detected as an electrical signal by the two - dimensional array - type x - ray detector 16 . an x - ray controller 34 supplies a trigger signal to a high - voltage generator 32 . the high - voltage generator 32 applies a high voltage to the x - ray source 14 upon receipt of the trigger signal . this causes the x - ray source 14 to apply the x - rays with the subject p . a top slit 26 is provided in the vicinity of the x - ray source 14 in order to prevent the subject p from being excessively exposed to the radiation . a top slit controller 36 is designed to control the width of the opening of the top slit 26 and is controlled by a system controller 40 , which will be described later . a base / bed controller 38 controls the rotation of the rotation ring 20 of the gantry 12 and the sliding movement of the sliding top of the bed 18 in synchronization with each other . the base / bed controller 38 may control either speeds or periods of the movements . the system controller 40 , which serves as the control center of the entire system , controls the x - ray controller 34 and the base / bed controller 38 in such a manner that the x - ray source 14 moves on a spiral track or in other words performs helical scanning when it is seen from the subject p . in particular , in this helical scanning operation , the rotation ring 20 continuously rotates at a certain angular speed and the sliding top moves at a certain speed so that x - rays are emitted from the x - ray source 14 either continuously or intermittently from angles which change at intervals of certain degrees . a memory unit 42 stores graphs and the like that indicate the relationship between the bed moving speed which will be described later and the number of collection rows of the detector . the stored data is read by the system controller 40 . a signal output by the two - dimensional array - type x - ray detector 16 is amplified for each channel and converted to a digital signal by a data collecting unit 46 . the projection data output by the data collecting unit 46 is input into a reconstruction processing unit 48 . the reconstruction processing unit 48 acquires back projection data to which an x - ray absorption rate is reflected for each voxel , based on the projection data . in the helical scanning method using x - ray cone beams as incorporated in the present embodiment , the tomography area ( the effective field of view ) forms a cylindrical shape with a radius of ω around the rotation axis . the reconstruction processing unit 48 defines voxels ( volume elements ) in this tomography area and finds the back projection data for each voxel . the three - dimensional image data or tomography data created from the back projection data is sent to a display device 50 and visually displayed as a three - dimensional image or a tomographic image thereon . the helical pitch represents the spacing of the spiral track of the x - ray source 14 , as illustrated in fig3 b . more specifically , the x - ray source 14 rotates 360 degrees along the surface around the center as indicated by an arrow in fig3 a . however , the sliding top moves during the 360 - degree rotation , and the distance of this movement is defined as a helical pitch . the 1pi / 3pi reconstruction is a typical example of the aforementioned complete - solution cone beam helical reconstruction . the data usage range is determined uniquely from the bed moving speed and the geometry . conversely , the width of the detector ( the collection width × the number of rows ) that is necessary for the image reconstruction can be calculated on the basis of the bed moving speed . by collecting the data in accordance with the calculation result ( the number of rows ), unnecessary exposure to the radiation can be avoided . according to the present embodiment , as a complete - solution cone beam helical construction ( katsevich reconstruction ) of a back projection type utilizing a filter , a structure of calculating the number or rows that is necessary for the optimal 1pi / 3pi reconstruction from any bed moving speed designated by the user and collecting data is conceived . in addition , the opening of the top slit is controlled in accordance with the selected number of rows of the detector so that an unnecessary amount of x - ray radiation can be avoided . hence , the complete - solution cone beam helical reconstruction can be realized without unnecessary radiation exposure with respect to any arbitrary bed moving speed . next , the operation of the x - ray ct device according to an embodiment of the present invention will be explained with reference to the flowchart of fig5 a . it is assumed here that the two - dimensional array - type x - ray detector 16 has 256 rows in the channel direction and the width of 0 . 5 mm in the direction of a row , as illustrated in fig4 . in the flowchart , in step s 1 , a collection range l is determined in accordance with a scanogram . then , in step s 2 , the collection time t is input and determined by the user . in step s 3 , the bed moving speed cs is calculated on the basis of steps s 1 and s 2 . in step s 4 , the number of collection rows of the detector is determined . for instance , a graph indicating the relationship between the bed moving speed ( cs ) and the number of collection rows ( nseg ) as shown in fig6 is stored in advance in the memory unit 42 to explain the relationship between katsevich reconstruction and the optimal number of rows , and therefore this graph is read from the memory unit 42 to determine the number of collection rows of the detector . in step s 5 , the width of the top slit 26 is controlled in accordance with the number of collection rows determined in step s 4 . in other words , the collection mode is determined . thereafter , the x - ray source 14 performs a scanning operation in step s 6 , and the projection data is thereby input to the data collecting unit 46 . in step s 7 , the reconstruction processing unit 48 conducts complete - solution cone beam helical reconstruction on the basis of the projection data obtained in step s 6 . in the above flowchart , the bed moving speed cs is calculated from the collection range l and the collection time t determined in steps s 1 and s 2 , but the process is not limited thereto . the bed moving speed cs may be input directly by the user . in step s 4 , the graph indicating the relationship between the bed moving speed and the number of collection rows is read from the memory unit 42 and referred to in order to determine the number of collection rows of the detector , but the process is not limited thereto . for instance , the determination may be made after the user inputs the aforementioned image mode ( high image quality mode / low radiation exposure mode ). when the user desires , for example , to perform data collection at a bed moving speed of 50 . 0 mm / rotation and image reconstruction in low radiation exposure mode ( 1pi ), the optimal collection condition is determined as 0 . 5 mm × 70 rows according to the graph of fig6 indicating the relationship between katsevich reconstruction ( 1pi / 3pi ) and the optimal number of rows ( when collecting at 0 . 5 mm ). when the user desires to perform image reconstruction in high image quality mode ( 3pi ), the optimal collection condition is 0 . 5 mm × 180 rows . thus , the data collection is performed on the ct device in accordance with this number . when collecting data , the top slit 26 is controlled in accordance with the optimal condition so as to adjust the opening to correspond to the condition of 0 . 5 mm × 70 rows or 0 . 5 mm × 180 rows . furthermore , the graph indicating the relationship between the bed moving speed and the number of collection rows of the detector as shown in fig6 may be displayed on the display device 50 . in place of the graph , a table may be provided to be referred to , or the condition may be calculated by a mathematical expression . the above embodiment includes two characteristic curves showing the relationship between the bed moving speed and the number of collection rows of the detector , for 1pi and 3pi . the curves are not limited thereto , however , and any modes , such as 5pi and 7pi , may be adopted as long as an image quality can be maintained without an artifact . moreover , the number of collection rows of the detector is figured out by calculating the bed moving speed as indicated by the flowchart of fig5 a according to the above embodiment , but the process is not limited thereto . for instance , the process may be conducted in accordance with the flowchart of fig5 b . more specifically , after a collection range l is determined in accordance with a scanogram in step s 11 , the bed moving period is determined by the user in step s 12 . then , the number of collection rows is determined in step s 13 . the subsequent processing operations in steps s 14 through s 16 are the same as steps s 5 through s 7 of fig5 a , and thus the explanation is omitted . as described above , the number of collection rows of the detector may be calculated on the basis of the bed moving speed . it should be noted that the present invention is not limited to the embodiments explained above , and that various modifications may be added without departing from the scope of the present invention . furthermore , the above embodiments include different steps of the invention , and thus various inventions can be attained from suitable combinations of disclosed structural elements . as long as the problems mentioned in the brief summary of the invention can be solved and the aforementioned advantages can be attained , the structure may be presented as an invention even if , for instance , some of the structural elements described in the embodiments are omitted or some of the structural elements are combined together . additional advantages and modifications will readily occur to those skilled in the art . therefore , the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein . accordingly , various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents .
an x - ray ct device according to the present invention produces an x - ray beam of a cone shape in the direction of the body axis of a subject from an x - ray source , performs a helical scanning operation in accordance with the relative movement and relative rotational motion of the x - ray source and the subject , processes data that is collected by a data collecting unit , and reconstructs an image by back - projecting the processed data at a reconstruction processing unit . when the moving speed of the bed is defined , a system controlling unit determines the optimal collection condition for a two - dimensional array - type x - ray detector .