Unfortunately, the temporal resolution of PET scanning isn’t high enough to use it to verify fMRI in most psychological tasks.
Here’s a figure that compares them directly on temporal resolution: http://www.nature.com/nature/journal/v468/n7321/fig_tab/nature09569_F1.html
Or alternatively, page 4 of this document: http://www.bus.umich.edu/neuroacrp/Yoon/Huettel.pdf has an even more detailed figure comparing spatial vs temporal resolutions of a larger number of neuroimaging techniques.
