[15O] H2O positron emission tomography (PET) has long been out of use in activation studies on the brain. Indeed, it is true that blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) has better spatial resolution and temporal resolution than PET, as well as no radiation exposure. However, PET and fMRI differ in their scope. Compared to fMRI, [15O] H2O PET offers advantages such as being quantifiable, less deteriorated by movement, and allowing for longitudinal studies. This article aimed to reassess the merits of PET in this context.