The increasing availability of MR/PET scanners opens new perspectives for cardiac imaging. However, technical issues for the reconstruction of signal from the moving myocardium remain to be solved. Using the k-space center in radial cardiac MRI data allows for generating a robust gating signal. Here, MR data acquired with a radial FLASH sequence in a 3 T MR/PET scanner continuously during a 15 minutes PET acquisition were used for retrospective gating. The k-space center magnitude was used to sort MRI and PET data into different cardiac phases while respiration-corrupted data were excluded, resulting in well separated cardiac phases.