Abstract: 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.