Method and apparatus for subsection analysis using computed tomography

Claim 1

In a system for image reconstruction by computed tomography wherein projection data signals are generated by detecting radiation beams projected through an internal area to be examined, said beams being projected in the plane of said area and at a plurality of scan angles relative thereto, said projection data signals representing the detected intensity of said beams, a method comprising the steps of: (a) storing said projection data signals; (b) storing a set of data values representing a filtering function; (c) reading from storage selected projection data signals generated by beams projected at a predetermined scan angle, said projection data signals being selected for readout under control of a designating signal defining a subsection area within said internal area to be examined such that only the projection data signals produced by beams passing through said subsection area and produced by beams passing through the regions immediately adjacent to and on either side of said subsection area are read out, said subsection area and said adjacent regions encompassing an area smaller than said internal area; (d) processing said signals read out from storage with said filtering function data values to produce a set of modified projection data signals; (e) storing said modified projection data signals; (f) repeating steps (c), (d) and (e) to generate and store additional modified projection data signals for beams projected at a plurality of different scan angles; and (b) accumulating in a memory pixel values representing the absorption coefficients for a plurality of points within said subsection area by reading from storage and back-projecting only those modified projection data signals corresponding to beams passing through said subsection area.