a fan beam of the type described herein is oriented so as to be coplanar with the cross - section of a body to be reconstructed . pure rotational movement is imparted to the fan beam by rotating the same about a fixed center of rotation such that the fan beam sweeps out an area in the plane of the cross - section ; and the absorption coefficients over such area can be computed in a known manner from measurements of the intensities of the beams emerging from the area swept out thereby . when one extremity of the fan beam passes through the fixed center of rotation , the area swept - out by the fan beam during one revolution thereof is circular , the center of which is coincident with the fixed center and the periphery of which is tangent to the other extremity of the fan beam . when the fixed center lies outside the fan beam , the area swept - out is annular and is defined by concentric circles whose centers are coincident with the fixed center and whose peripheries are respectively tangent to the extremities of the fan beam . by sequentially rotating the fan beam about the fixed center , and shifting the fan beam relative thereto after each revolution , a circular area of arbitrary size can be swept - out by the fan beam after a number of revolutions . the size of such area is thus independent of the apical angle of the fan beam .