Simultaneous trimodal EEG-PET-MR imaging can be a tool to delve deeper in the healthy human brain functioning. One of the aspects scarcely investigated is the interplay between HD-EEG net and PET signal detection and reconstruction. In this work, we assess the impact of HD-EEG on both attenuation corrected (AC) and non-attenuation corrected (NAC) PET images. We found that HD-EEG net attenuates PET signal in NAC images in both phantom and human brain, respectively of 3.01%(±3.69%) and of 0.53%(±5.89%). AC images show an opposite behaviour: attenuation correction factor tends to be overestimated in CT- and UTE-derived UMAP obtained with HD-EEG net.