The correct answer is PET requires a radioactive tracer.
In actuality, PET has a higher signal-to-noise ratio (SNR) than fMRI for a single scan. However, because there is no radiation exposure, fMRI generally produces a crisper image and can be done numerous times. Indeed, this may be a sufficient explanation for the shift toward fMRI from PET, which has been on the decline.
[15O] The use of H2O PET in brain activation research has long since been discontinued. There is no radiation exposure, and functional magnetic resonance imaging (fMRI) with blood oxygen level-dependent (BOLD) has a superior spatial and temporal resolution to PET.
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