Attenuation correction or fusion: The practice opposed to theory

Positron emission tomography (PET) has gained general acceptance in the practice of clinical oncology. The recently introduced use of sequential PET and CT in the same bed permits parallel acquisition of functional and detailed anatomical data. The automatic co-registration of the two sets of data permits the improvement of diagnostic accuracy of this bimodal system and facilitates the diagnostic procedure. At the same time it reduces the acquisition time for PET studies, permitting better patient compliance and lower noise from emission data in the attenuation correction process of PET studies. But the use of CT transmission data generated by a wide range of X-ray low energy photons in some cases produces important artifacts on the attenuation corrected PET images due to the different absorption of the X-ray and annihilation of monochromatic photons by the body structures of different densities. Moreover the acquisition time of CT data is very small in comparison to the acquisition time of PET data, producing motion artifacts related to respiratory cycle or to voluntary movements of the patient.