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

Abstract.

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 current 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 PET_CT10–PET_CT120. In 60 tumorous lesions, FDG uptake and lesion size were quantified on PET_CT10–PET_CT120. In another group of 18 patients, one CT scan acquired with 80 mA and a standard transmission scan acquired using ⁶⁸Ge sources were employed for the attenuation correction of the FDG emission scan (PET_CT80, PET_68Ge). Uptake values and lesion size in 26 lesions were compared on PET_CT80 and PET_68Ge. 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 ⁶⁸Ge attenuation correction, especially in lesions with high FDG uptake. Lesion size was similar on PET_CT80 and PET_68Ge. In conclusion, low CT currents yield adequate maps for the attenuation correction of PET emission scans. Although the discrepancy between CT- and ⁶⁸Ge-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.