European Journal of Nuclear Medicine Vol. 29, No. 3, March 2002 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 (CT 10 , CT 40 , CT 80 and CT 120 ) 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 le- sion size were quantified on PET CT10 –PET CT120 . In an- other group of 18 patients, one CT scan acquired with 80 mA and a standard transmission scan acquired using 68 Ge sources were employed for the attenuation correc- tion of the FDG emission scan (PET CT80 , PET 68Ge ). Up- take 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 sec- ond group, tumour FDG uptake was slightly higher using CT compared with 68 Ge attenuation correction, especial- ly in lesions with high FDG uptake. Lesion size was sim- ilar on PET CT80 and PET 68Ge . In conclusion, low CT cur- rents yield adequate maps for the attenuation correction of PET emission scans. Although the discrepancy be- tween CT- and 68 Ge-derived uptake values is probably not relevant in most cases, it should be kept in mind if standardised uptake values derived from CT and 68 Ge at- tenuation correction are compared. Keywords: Positron emission tomography – Computed tomography – FDG – Tumour staging Eur J Nucl Med (2002) 29:346–350 DOI 10.1007/s00259-001-0698-9 Introduction Over recent years, positron emission tomography (PET) has proved to be an excellent imaging modality, particu- larly for tumour staging and therapy monitoring. In PET scans, attenuation correction is mandatory if tracer up- take is to be quantified. Furthermore, it has been shown that lesion geometry is often distorted if one uses non-at- tenuation-corrected emission data only [1]. Most com- monly, attenuation correction is performed using rotating germanium-68 pin sources. A disadvantage of this ap- proach is the low photon flux of these sources, which re- sults in long acquisition times for the transmission scan and poor transmission scan image quality, and hence lit- tle anatomical information. Depending on the required quality and the imaging protocol, a transmission scan takes from 2 to 15 min per field of view (FOV). For a whole-body scan this may add up to 30 min, meaning in- creased patient discomfort and lower patient throughput [2]. A major improvement in the above limitations can be expected from the recent introduction of combined PET/CT systems. In these systems, the CT acquires im- ages with higher resolution and greater contrast in a much shorter scan time than the 68 Ge transmission system. In addition to replacing 68 Ge scans for attenua- tion correction, the CT scans provide accurate anatomi- cal delineation of pathologies which can be co-registered with the PET scan [3, 4]. With the introduction of these new combined systems, several questions need to be an- swered. One concerns the optimal X-ray tube current for the acquisition of the CT scan. High currents improve image quality but also increase radiation dose to the pa- Alfred Buck ( ✉ ) PET Center, Division of Nuclear Medicine, Department of Radiology, University Hospital, 8091 Zürich, Switzerland e-mail: buck@nuklearmed.unizh.ch Tel.: +41-1-2551111, Fax: +41-1-2554414 Original article CT vs 68 Ge attenuation correction in a combined PET/CT system: evaluation of the effect of lowering the CT tube current Ehab Kamel 1 , Thomas F. Hany 1 , Cyrill Burger 1 , Valerie Treyer 1 , Albert H. R. Lonn 2 , Gustav K. von Schulthess 1 , Alfred Buck 1 1 PET Center, Division of Nuclear Medicine, Department of Radiology, University Hospital, 8091 Zürich, Switzerland 2 GE Medical Systems Received 6 August and in revised form 14 October 2001 / Published online: 18 December 2001 © Springer-Verlag 2001