CT vs 68Ge attenuation correction in a combined PET/CT system:: evaluation of the effect of lowering the CT tube current

With the introduction of combined positron emission tomography/computed tomography (PET/CT) systems, several questions have to be answered. In this work we addressed two of these questions: (a) to what value can the CT tube cut-rent be reduced while still yielding, adequate maps for the attenuation correction of PET emission scans and (b) how do quantified uptake values in tumours derived from CT and germanium-68 attenuation correction compare. In 26 tumour patients, multidetector CT scans were acquired with 10 40, 80 and 120 mA (CT10, CT40, CT80, and CT120) and used for the attenuation correction of a single FDG PET emission scan. yielding four PET scans designated PETCT10-PETCT120. In 60 tumorous lesions. FDG uptake and lesion size were quantified on PETCT10-PETCT120. In another group of 18 patients. one CT scan acquired with 80 mA and a standard transmission scan acquired using Ge-68 sources were employed for the attenuation correction of the FDG emission scan (PETCT80, PET68Ge). Uptake values and lesion size in 26 lesions were compared on PETCT80 and PET68Ge. In the first group of patients, analysis of variance revealed no significant effect of CT current on tumour FDG uptake or lesion size. In the second group, tumour FDG uptake was slightly higher using CT compared with Ge-68 attenuation correction. especially in lesions with high FDG uptake. Lesion size was similar on PETCT80 and PET68Ge. In conclusion, low CT currents yield adequate maps for the attenuation correction of PET emission scans. Although the discrepancy between CT- and Ge-68-derived uptake values is probably not relevant in most cases. it should be kept in mind if standardised uptake values derived from CT and 68Ge attenuation correction are compared.