Name a imaging modality that primarily measures metabolic function.

• A. MRI
• B. PET
• C. CT
• D. fMRI

Answer: (B)

Imaging metabolic function means measuring how much energy cells are using, not just where they are or how blood flows. PET does this directly by using radiotracers that participate in metabolism, most commonly FDG, a glucose analog. Cells with higher metabolic activity take up more tracer and trap it, so the PET image maps regions of higher metabolic rate across tissues. This direct readout of metabolic activity sets PET apart from other modalities: MRI and CT primarily show anatomy, not metabolism, while fMRI tracks changes in blood flow and oxygenation as a proxy for neural activity, which is an indirect measure of metabolism. So PET is the best choice for imaging metabolic function.