Which option describes what fMRI registers?

• A. Structural anatomy only
• B. Blood flow to functioning areas of the brain
• C. Electrical impulses
• D. Metabolic rate of glucose

Answer: (B)

fMRI measures changes in blood flow to active brain areas, using the BOLD signal as an indirect index of neural activity. When a region is engaged, neurons fire more and consume more oxygen, prompting local blood vessels to increase blood flow. This overcompensates for oxygen use, changing the balance of oxyhemoglobin and deoxyhemoglobin, which alters the magnetic resonance signal detected by the scanner. That makes fMRI map where activity is happening across the brain.

This is different from structural MRI, which captures anatomy rather than function. It also isn’t measuring electrical impulses directly—that’s what EEG/MEG do. And it doesn’t directly measure glucose metabolism—that’s what PET with radiotracers like FDG assesses. So the description of blood flow to functioning areas best captures what fMRI registers.