CT-based attenuation correction for PET/CT scanners in the presence of contrast agent

In PET/CT tomographs the PET emission data can be corrected for photon attenuation effects using the CT images which are scaled from the mean CT energy of /spl sim/70 keV to the PET photon energy of 511 keV. The standard scaling approach requires segmenting the CT image voxels into soft tissue or bone using a single threshold of 300 H.U. and then applying two separate scale factors. In practice oral contrast may be administered leading to enhanced CT values that are not correctly handled by the standard scaling algorithm, which then leads to a bias in the PET emission image corrected for attenuation. A modified algorithm is proposed which transforms the oral contrast enhanced CT values to the correct 511 keV value, leading to unbiased attenuation correction factors.