  Tomographic Gamma Scanning (TGS) is a technique used to assay the nuclide
distribution and radioactivity in nuclear waste drums. Both transmission and
emission scans are performed in TGS and the transmission image is used for the
attenuation correction in emission reconstructions. The error of the
transmission image, which is not considered by the existing reconstruction
algorithms, negatively affects the final results. An emission reconstruction
method based on Noise Equivalent Counts (NEC) is presented. Noises from the
attenuation image are concentrated to the projection data to apply the NEC
Maximum-Likelihood Expectation-Maximization algorithm. Experiments are
performed to verify the effectiveness of the proposed method.
