Patent Number: 
Section: claims

1. A CT image producing method comprising the steps of: collecting first axial scan data while rotating at least one of an X-ray tube and a multi-row detector relative to and around a subject to be imaged at a first position in a direction of a body axis of the subject to be imaged; extracting from said first axial scan data projection data corresponding to a plurality of reconstruction fields arranged in the direction of the body axis of the subject to be imaged; if part of projection data of a reconstruction field near an end of said multi-row detector cannot be extracted from said first axial scan data, collecting second axial scan data while rotating at least one of said X-ray tube and multi-row detector relative to and around the subject to be imaged at a second position to which said X-ray tube and multi-row detector are rectilinearly moved relative to the subject to be imaged from said first position toward the end of said multi-row detector; extracting from said second axial scan data the projection data that could not be extracted from said first axial scan data of the reconstruction field near the end of said multi-row detector; and producing one CT image based on said extracted projection data of the reconstruction fields. 2. The CT image producing method of claim 1, wherein said second position is a position distant from said first position by a cone beam width at a center of rotation. 3. The CT image producing method of claim 1, further comprising the steps of: determining combined projection data by applying weighted addition to said extracted projection data across the reconstruction fields; and producing one CT image from said combined projection data. 4. The CT image producing method of claim 3, wherein a weight for the weighted addition on said projection data is determined from a mutual relationship between the reconstruction fields, and from a predetermined weighting function. 5. The CT image producing method of claim 1, further comprising the steps of: producing respective CT images from said extracted projection data of the reconstruction fields; and applying weighted addition to said CT images to produce one CT image. 6. The CT image producing method of claim 5, wherein a weight for the weighted addition on said CT images is determined from a mutual relationship between the reconstruction fields, and from a predetermined weighting function. 7. The CT image producing method of claim 3, further comprising the steps of: extracting from said first axial scan data projection data corresponding to one line or a plurality of parallel lines at spacings of a plurality of pixels on each of said reconstruction planes; if part of projection data of a reconstruction field near an end of said multi-row detector cannot be extracted from said first axial scan data, extracting the projection data from said second axial scan data; generating projection line data by multiplying said projection data by a cone beam reconstruction weight; determining combined projection line data by applying weighted addition to said projection line data of corresponding lines across the reconstruction fields; generating image-positional line data by filtering said combined projection line data; determining backprojected pixel data of pixels on an image plane based on said image-positional line data; determining backprojected data by adding the backprojected pixel data on a pixel-by-pixel basis across all views used in image reconstruction; and thereby producing one CT image. 8. The CT image producing method of claim 5, further comprising the steps of: extracting from said first axial scan data projection data corresponding to one line or a plurality of parallel lines at spacings of a plurality of pixels on each of said reconstruction planes; if part of projection data of a reconstruction field near an end of said multi-row detector cannot be extracted from said first axial scan data, extracting the projection data from said second axial scan data; generating projection line data by multiplying said projection data by a cone beam reconstruction weight; generating image-positional line data by filtering said projection line data; determining backprojected pixel data of pixels on an image plane based on said image-positional line data; determining backprojected data by adding the backprojected pixel data on a pixel-by-pixel basis across all views used in image reconstruction; applying weighted addition to the corresponding backprojected data across said reconstruction fields; and producing one CT image. 9. An X-ray CT apparatus comprising: an X-ray tube; a multi-row detector; a first axial scanning device for collecting first axial scan data while rotating at least one of said X-ray tube and multi-row detector relative to and around a subject to be imaged at a first position in a direction of a body axis of the subject to be imaged; a first projection data extracting device for extracting from said first axial scan data projection data corresponding to a plurality of reconstruction fields arranged in the direction of the body axis of the subject to be imaged; a second axial scanning device for, if part of projection data of a reconstruction field near an end of said multi-row detector cannot be extracted from said first axial scan data, collecting second axial scan data while rotating at least one of said X-ray tube and multi-row detector relative to and around the subject to be imaged at a second position to which said X-ray tube and multi-row detector are rectilinearly moved relative to the subject to be imaged from said first position toward the end of said multi-row detector; a second projection data extracting device for extracting from said second axial scan data the projection data that could not be extracted from said first axial scan data of the reconstruction field near the end of said multi-row detector; and a CT image producing device for producing one CT image based on said extracted projection data of the reconstruction fields. 10. The X-ray CT apparatus of claim 9, wherein said second position is a position distant from said first position by a cone beam width at a center of rotation. 11. The X-ray CT apparatus of claim 9, wherein said CT image producing device determines combined projection data by applying weighted addition to said extracted projection data across the reconstruction fields, and produces one CT image from said combined projection data. 12. The X-ray CT apparatus of claim 11, wherein a weight for the weighted addition on said projection data is determined from a mutual relationship between the reconstruction fields, and from a predetermined weighting function. 13. The X-ray CT apparatus of claim 9, wherein said CT image producing device produces respective CT images from said extracted projection data of the reconstruction fields, and applies weighted addition to said CT images to produce one CT image. 14. The X-ray CT apparatus of claim 13, wherein a weight for the weighted addition on said CT images is determined from a mutual relationship between the reconstruction fields, and from a predetermined weighting function. 15. The X-ray CT apparatus of claim 11, wherein said first projection data extracting device extracts from said first axial scan data projection data corresponding to one line or a plurality of parallel lines at spacings of a plurality of pixels on each of said reconstruction planes; said second projection data extracting device extracts from said second axial scan data the projection data that could not be extracted from said first axial scan data of a reconstruction field near an end of said multi-row detector; and said CT image producing device generates projection line data by multiplying said projection data by a cone beam reconstruction weight, determines combined projection line data by applying weighted addition to said projection line data of corresponding lines across the reconstruction fields, generates image-positional line data by filtering said combined projection line data, determines backprojected pixel data of pixels on an image plane based on said image-positional line data, and determines backprojected data by adding the backprojected pixel data on a pixel-by-pixel basis across all views used in image reconstruction. 16. The X-ray CT apparatus of claim 13, wherein said first projection data extracting device extracts from said first axial scan data projection data corresponding to one line or a plurality of parallel lines at spacings of a plurality of pixels on each of said reconstruction planes; said second projection data extracting device extracts from said second axial scan data the projection data that could not be extracted from said first axial scan data of a reconstruction field near an end of said multi-row detector; said CT image producing device generates projection line data by multiplying said projection data by a cone beam reconstruction weight, generates image-positional line data by filtering said projection line data, determines backprojected pixel data of pixels on an image plane based on said image-positional line data, determines backprojected data by adding the backprojected pixel data on a pixel-by-pixel basis across all views used in image reconstruction, applies weighted addition on the corresponding backprojected data across said reconstruction fields, and produces one CT image.