Abstract: A method is disclosed for obtaining linear attenuation coefficients for interpreting a PET scan of a region. The method is suitable for use when high molecular weight materials are present in the region, such as contrast agents or metal objects. The method includes obtaining first and second x-ray CT data sets of the region of interest at two different energies or voltage potentials and differencing corresponding CT numbers. The difference values are used to distinguish portions of the region that are bone from portions of the region that are contrast agent or other high molecular weight material. The obtained CT data set is then used to obtain an attenuation coefficient map of the region at the PET energy of 511 keV, for example, using a linear scaling factor suited to the particular identified material. Difference values at or near zero may be used to identify soft tissue portions of the region.