The use of CT data instead of beta+ sources transmission scan in PET-CT, has been validated, so far, in terms of accuracy, precision and dose only for whole-body studies. Aim of this study was to compare the two alternative methods, in brain studies, taking into account the following parameters: the systematic error of the attenuation coefficients, introduced by the conversion function used by the scanner; the noise on the measure depending on the CT tube current; the noise on the measure carried-out with the external sources in different experimental conditions.