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Abstract. Molecular imaging is usually based on the recognition by the radiopharmaceuticals of specific sites which are present in limited number or density in the cells or biological tissues. Thus is of high importance to label the radiopharmaceuticals with high specific activity to be able to achieve a high target to non target ratio. The presence of carbon dioxide (CO 2 ) from the air containing 98,88% of 12 C and 1,12% 13 C compete with 11 CO 2 produced at the cyclotron. In order to minimize the presence of these isotopes along the process of irradiation, transferring and synthesis of radiopharmaceuticals labelled with 11 C, we applied this method: previous to the irradiation the target was 3-4 times flushed with He (5.7) as a cold cleaning, followed by a similar conditioning of the line, from the target up to the module, and finally a hot cleaning in order to desorb 12 CO 2 and 13 CO 2 , this was performed by irradiation during 1 min at 5 uA (3 times). In addition, with the aim of improving quality of gases in the target and in the modules, water traps (Agilent) were incorporated in the inlet lines of the target and modules. Target conditioning process (cold and hot flushings) as well as line cleaning, allowing the desorption of unlabelled CO 2 , together with the increasing of gas purity in the irradiation and in the synthesis, were critical parameters that enable to achieve 11 C-radiopharamaceuticals with high specific activity, mainly in the case of 11 C-PIB.

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