We are investigating the utility of X-ray computed tomography (CT) generated attenuation coefficient maps for compensation of photon attenuation in SPECT images. An attenuation coefficient distribution generated by X-ray CT with a specific energy spectrum must be accurately translated to the energy of the radionuclide used in SPECT. To study this translation, we have developed a realistic 3D thorax phantom which is used to simulate both emission and transmission projection data for a given monoenergetic or polyenergetic (X-ray) source within the range 1 to 1000 keV. Transmission projection data are simulated from the 3D phantom for a standard X-ray spectrum and fan-beam geometry. The projection data are reconstructed to form an X-ray CT image which is then translated to an estimate of the attenuation coefficient map at the energy of the emission radionuclide. Finally, the SPECT images are reconstructed using the maximum-likelihood expectation-maximization (ML-EM) algorithm and attenuation maps obtained from the translated CT image. Other attenuation maps were also used in the investigation. The reconstructed images are compared to each other and the original emission phantom. The preliminary results are important in developing and assessing further methods to translate X-ray CT images for attenuation compensation in SPECT.