an embodiment of the present invention will be described below with reference to the views of the accompanying drawing . the following is an outline of a processing procedure in this embodiment . 1 ) imaging is repeated while a c - arm is rotated , accompanied by electrocardiographic measurement of an object . 2 ) a plurality of relatively rough three - dimensional images ( 3d volume data ) respectively corresponding to a plurality of cardiac phases are reconstructed by a iterative reconstruction method . 3 ) a point in a real space which is anatomically identical to a target point on a relatively rough three - dimensional image is calculated ( positional shift calculation ). 4 - 1 ) a plurality of relatively fine three - dimensional images respectively corresponding to a plurality of cardiac phases are reconstructed by analytic reconstruction processing , accompanied by correction of back projection loci based on positional shifts . the processing 4 - 1 ) described above can be replaced by processing 4 - 2 ) and processing 4 - 3 ). 4 - 2 ) a plurality of relatively fine three - dimensional images respectively corresponding to a plurality of cardiac phases are reconstructed by analytic reconstruction processing . 4 - 3 ) a plurality of three - dimensional images shifted based on positional shifts are combined . the following is an outline of a processing procedure by a modification of this embodiment . 1 ) imaging is repeated while the c - arm is rotated , accompanied by electrocardiographic measurement of an object . 2 ) a plurality of three - dimensional images ( 3d volume data ) respectively corresponding to a plurality of cardiac phases are reconstructed by a iterative reconstruction method or a filtered back projection method . 3 ) a point in a real space which is anatomically identical to a target point on a three - dimensional image is calculated ( positional shift calculation ). 4 ) a plurality of three - dimensional images respectively corresponding to a plurality of cardiac phases are reconstructed by the filtered back projection method . 5 ) a plurality of three - dimensional images are shifted based on positional shifts . 6 ) a plurality of combined images are generated by combining a plurality of three - dimensional images while a bias shift pattern is changed . the bias shift pattern is a set of the estimated values of positional shifts corresponding to a plurality of cardiac phases . the estimated values of positional shifts are changed for each of the x -, y -, and z - axes pixel by pixel within the range of − 3 pixels to + 3 pixels . in processing 7 ) described next , a combined image corresponding to a bias shift pattern substantially exhibiting the minimum positional shift is selected . 7 ) a combined image exhibiting the minimum positional shift is selected as a final image from a plurality of combined images ( fine position correction ). a reconstructed image in each cardiac phase comprises a small number of projection data , which cover reconstruction area sparsely . that is , this image is reconstructed from incomplete projection data . the incompleteness of projection data causes artifacts . artifacts occur on the positive side and the negative side . positive - side artifacts and negative - side artifacts alternately occur radially from the reconstruction center . the positions where positive - side artifacts and negative - side artifacts appear gradually move due to the influences of the pulsation of the heart . therefore , the degree of a positional shift appears as the total sum of negative reconstruction values in a combined image . a combined image exhibiting the minimum total sum ( absolute value ) of negative reconstruction values is selected as a final three - dimensional image exhibiting the minimum positional shift . reconstruction values may be replaced ct values , the absorption coefficient values , or the absorption coefficient values multiplied a constant coefficient . as shown in fig1 and 2 , a gantry 1 of an x - ray diagnosis apparatus according to this embodiment includes a c - shaped arm 102 which holds an x - ray tube 2 which generates x - rays in a cone beam shape upon receiving a high voltage from a high voltage generator ( not shown ), together with a two - dimensional x - ray detector 5 typically serving as an fpd ( flat panel detector ) which detects x - rays transmitted through an object . a data acquisition unit 11 ( not shown ) acquires the data of two - dimensional projection images ( to be simply referred to as two - dimensional images hereinafter ) via the x - ray detector 5 . an image memory 12 stores the data of two - dimensional images . the c - shaped arm 102 forms , together with a hanger arm 101 , a support mechanism ( not shown ). the support mechanism is equipped with an electric motor , and can rotate the c - shaped arm 102 at a constant speed under the control of a control unit . in rotational imaging in step s 11 shown in fig3 , the c - shaped arm 102 is rotated at a constant speed to continuously generate x - rays , and two - dimensional image data are repeatedly acquired via the data acquisition unit 11 in a predetermined cycle . for example , repeatedly acquiring data at 1 ° intervals while rotating the c - shaped arm 102 around a patient within the range of an angle of 200 ° will acquire 200 - frame two - dimensional images corresponding to 200 ° with a view pitch of 1 °. the image memory 12 stores the data of a plurality of two - dimensional images acquired by the data acquisition unit 11 . the data of a cardiac phase at each imaging operation based on an electrocardiographic waveform from an electrocardiograph ( ecg ) 3 is associated with each of a plurality of two - dimensional images . a filtering unit 13 and an affine transform unit 14 perform preprocessing such as high frequency enhancement filtering and image enlargement / movement for two - dimensional images . a lookup table ( lut ) 15 converts the tone of an image to be displayed . a three - dimensional image processing unit 16 has a function of converting three - dimensional image data to be displayed into a two - dimensional display image by volume rendering processing including projection processing . a two - dimensional image processing unit 22 performs subtraction processing for enhancing a contrast - enhanced blood vessel by subtracting two - dimensional images , of the two - dimensional images stored in the image memory 12 , which correspond to view angles before and after the injection of a contrast medium , background suppression processing for enhancing a contrast - enhanced blood vessel by using only a contrast - enhanced in the same manner , and density irregularity correction processing . in the background suppression processing in step s 12 shown in fig3 , for example , an image is generated by applying a low - pass filter to each two - dimensional image , and a corresponding original frame image is subtracted from each filtered image . note that subtraction may be performed for the data obtained by adding and averaging data corresponding to about one electrocardiographic cycle centered on each frame . in addition , existing background subtraction processing may be applied to the above data . in step s 13 shown in fig3 , a iterative three - dimensional reconstruction processing unit 19 reconstructs the data of a plurality of relatively rough three - dimensional images ( also called volume data ) respectively corresponding to a plurality of cardiac phases as shown in fig4 by performing iterative three - dimensional reconstruction processing such as the art method or map - em method for the overall data acquisition range based on the data of a plurality of two - dimensional images at different view angles ( imaging angles ) which have undergone background suppression processing . the data acquisition range is defined as a cylinder inscribed in an x - ray beam emitted from the x - ray tube 2 in all directions . in this embodiment , the iterative three - dimensional reconstruction processing unit 19 reconstructs the data of a plurality of three - dimensional images respectively corresponding to a plurality of cardiac phases . more specifically , data having undergone background suppression processing is successively approximately optimized according to equation ( 1 ) given below : where h is a projection matrix , f t is the data obtained by aligning the respective voxels of a reconstructed image in a given cardiac phase t to a column vector , g t is the data obtained by aligning the projection data having undergone background subtraction processing in the given cardiac phase t to a column vector in the order of the respective pixels and frames , and a is α constant number . obtaining f t that minimizes e will complete the reconstruction . this processing is repeated for n cardiac phases t . as a result , the data of n three - dimensional images are generated . although a method with a noise suppression effect ( equation 1 ) has been described as a reconstruction method , the present invention is not limited to this . it is possible to use other iterative reconstruction methods ( e . g ., map - em and tv ). in step s 14 in fig3 , a positional shift detection unit 21 specifies a position ( three - dimensional coordinates ) in a real space which is anatomically identical to a region ( target point ) from each of a plurality of relatively rough three - dimensional images which are reconstructed by the iterative three - dimensional reconstruction processing unit 19 and respectively correspond to a plurality of cardiac phases . one of a plurality of specified target points is set as a reference position , and correction vectors associated with n − 1 target points are calculated with reference to the reference position . in practice , as shown in fig6 , a three - dimensional image ( reference image ) in a predetermined one cardiac phase is selected from n reconstructed three - dimensional images . the three - dimensional image processing unit 16 performs volume rendering processing for the reference image . a display unit 18 displays the volume - rendered image . the operator designates the center ( target point ) of a target region with an input device such as a mouse ( not shown ). typically , a target region is a contrast - enhanced blood vessel , and a target point to be designated is a characteristic region such as a stenotic portion or blood vessel branch portion . the coordinates of the designated target point are sent to a target region identifying unit 23 . a projection locus in volume rendering processing which corresponds to the target point is specified . the intersection between the projection locus and the contrast - enhanced blood vessel portion is identified . a blood vessel branch in a local range centered on the intersection is specified as an initial target point . the data of the n reconstructed three - dimensional images and the coordinate data of the initial target point are sent to the positional shift detection unit 21 . as shown in fig7 , the positional shift detection unit 21 extracts a target region in the same blood vessel portion by searching for a predetermined range centered on the initial target point on a three - dimensional image in another cardiac phase adjacent to the cardiac phase corresponding to the initial target point , and specifies the center of the extracted target region as a target point in the cardiac phase . the positional shift detection unit 21 repeats this processing in all cardiac phases . the positional shift detection unit 21 specifies a target region with respect to an adjacent cardiac phase in the same manner , and specifies the center of the target region as a new target point in the adjacent cardiac phase . the positional shift detection unit 21 sequentially specifies target points in all the cardiac phases . the positional shift detection unit 21 calculates a correction vector for aligning the coordinates of the target points in all the cardiac phases to the same coordinates . typically , the positional shift detection unit 21 calculates a correction vector so as to align the coordinates of the target point in another cardiac phase to the coordinates of the target point in the cardiac phase at an middle - diastolic phase . in step s 16 in fig3 , an analytic three - dimensional reconstruction processing unit 20 reconstructs relatively fine three - dimensional image data ( see fig5 ) by performing analytic three - dimensional reconstruction processing such as the filtered back projection method , back projection filter method , convolution method , or fourier transform method based on the data of a plurality of two - dimensional images at different view angles which have undergone subtraction ( step s 15 ) of a density irregularity correction image by the two - dimensional image processing unit 22 . density irregularity correction ( non - uniformity correction ) is performed by subtracting two - dimensional images from air calibration images . the following description is based on the assumption that the filtered back projection method proposed by feldkamp et al ., which can obtain good image quality , is used . according to the feldkamp method , first of all , 200 subtraction images are subjected to proper convolution filtering such as shepp & amp ; logan or ramachandran . the analytic three - dimensional reconstruction processing unit 20 then shifts a back projection locus in accordance with a correction vector in each cardiac phase . back projection processing is executed in accordance with the shifted back projection locus . likewise , the analytic three - dimensional reconstruction processing unit 20 then shifts a back projection locus for each cardiac phase in accordance with a correction vector in a corresponding cardiac phase . the analytic three - dimensional reconstruction processing unit 20 executes back projection processing in accordance with the shifted back projection loci . this reconstructs a three - dimensional image with a reduced positional shift . in general , a reconstruction region is defined as a cylinder inscribed in an x - ray beam emitted in all directions . in this case , as shown in fig5 , a reconstruction region is a region much narrower than a general reconstruction region centered on the position of a stenotic portion which is designated by the operator . this reconstruction region is preferably a cube of about 3 cm × 3 cm × 3 cm or a smaller region . when designating a stenotic portion , the operator may designate an approximate range on the portion . back projection computation is generally performed in accordance with a projection system defined by an x - ray optical system . in this case , however , as shown in fig8 , reconstruction is performed upon motion correction by performing back projection computation so as to correct the motion . fig8 shows an example of the reconstruction of the position of a marker . assume that the position designated by the operator is a diastolic phase , and the current position moves as shown in fig8 . normally , back projection should be performed in accordance with an x - ray locus passing through the current position . however , since the normal position is the position of a diastolic phase , back projection computation is performed such that the locus of the position passes through the same target point and is parallel to a locus passing through the current position . motion correction is performed in the same manner assuming approximations at positions other than a target point . note that a back projection locus is shifted in accordance with a correction vector . however , it is possible to shift each of a plurality of three - dimensional images reconstructed for each cardiac phase in accordance with a correction vector and combine the shifted images . the three - dimensional image processing unit 16 converts the three - dimensional image reconstructed by the analytic method into an image for display by using a method such as volume rendering . the display unit 18 displays this image . in the above embodiment , a single point is designated as a target point . however , a plurality of points may be designated . data within a predetermined range from a designated point is reconstructed and displayed . in the above embodiment , a target point is designated . however , for example , it is possible to identify a global positional shift upon executing the processing of reducing the resolution while holding branch information and to identify fine positional shift information while sequentially improving the resolution . performing this processing will eliminate the necessity to designate corresponding points and can reconstruct an overall region at once . in the above embodiment , a target point is designated . if , however , for example , a device having a gps function exists near a target region , position correction may be performed based on the information obtained by the gps function . in this case , since the three - dimensional information of the device can be used , there is no need to perform tracking . fig9 shows a processing procedure by a modification of this embodiment . the same reference numerals as in fig9 and 3 denote the same steps . first of all , this apparatus repeats imaging during the rotation of the c - arm , accompanied by electrocardiographic measurement of an object ( s 11 ). the apparatus then reconstructs a plurality of three - dimensional images ( 3d volume data ) respectively corresponding to a plurality of cardiac phases from a plurality of projection images having undergone background suppression processing ( s 12 ) by using the iterative reconstruction method or filtered back projection method ( s 13 ), and identifies a positional shift for each phase ( s 14 ). the analytic three - dimensional reconstruction processing unit 20 reconstructs a plurality of three - dimensional images respectively corresponding to the plurality of cardiac phases from the plurality of projection images having undergone density irregularity correction processing ( s 15 ) by using the filtered back projection method ( s 25 ). the analytic three - dimensional reconstruction processing unit 20 or another position correction processing unit corrects each of the positions of the plurality of three - dimensional images in accordance with the positional shifts identified in step s 14 ( s 26 ). the analytic three - dimensional reconstruction processing unit 20 or another position correction processing unit generates a plurality of combined images by repeatedly combining the plurality of corrected three - dimensional images ( s 27 ). the plurality of combined images differ in corresponding bias shift patterns . a bias shift pattern is a set of the estimated values of positional shifts relative to a plurality of cardiac phases . the estimated values of positional shifts are changed for each of the x -, y -, and z - axes pixel by pixel within , for example , the range of − 3 pixels to + 3 pixels . using an enormous number of bias shift patterns will generate an enormous number of combined images with the estimated values of positional shifts being slightly different from each other . the analytic three - dimensional reconstruction processing unit 20 or another position correction processing unit selects a single combined image exhibiting the minimum positional shift from the enormous number of combined images with the estimated values of the positional shifts being slightly different from each other ( s 28 ). with this processing , very fine position correction is performed . a reconstructed image in each cardiac phase comprises a small number of projection data , which cover reconstructuon area sparsely . that is , this image is reconstructed from incomplete projection data . the incompleteness of projection data causes artifacts . artifacts occur on the positive side and the negative side . positive - side artifacts and negative - side artifacts alternately occur radially from the reconstruction center . the positions where positive - side artifacts and negative - side artifacts appear gradually move due to the influences of the pulsation of the heart . therefore , the degree of a positional shift appears as the total sum ( absolute value ) of negative ct values or image values , that is absorption coefficient multiplied by constant coefficient in a combined image . negative ct values and negative image values are same in the case of soft - tissue region and air - region , respectively . a combined image exhibiting the minimum total sum of negative reconstruction values is selected as a final three - dimensional image exhibiting the minimum positional shift . while certain embodiments have been described , these embodiments have been presented by way of example only , and are not intended to limit the scope of the inventions . indeed , the novel methods and systems described herein may be embodied in a variety of other forms ; furthermore , various omissions , substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions . the accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions .
according to one embodiment , an x - ray diagnosis apparatus comprises a c - arm , a support mechanism rotatably supporting the c - arm , a rotation driving unit driving rotation of the c - shaped arm , an x - ray tube mounted on the c - arm , an x - ray detector mounted on the c - arm , a control unit controlling the x - ray tube and the x - ray detector , a storage unit storing two - dimensional images generated , a first reconstruction processing unit reconstructing first three - dimensional images respectively corresponding to a plurality of cardiac phases based on acquired two - dimensional images , a positional shift amount calculation unit calculating positional shift amounts respectively corresponding to cardiac phases based on the first three - dimensional images , and a second reconstruction processing unit reconstructing a single second three - dimensional image with a reduced positional shift based on the plurality of acquired two - dimensional images by using the calculated positional shift amounts .