FMRI is a technique that measures brain activity by detecting changes in blood flow and oxygenation levels in the brain in response to neural activity. It has both advantages and disadvantages compared to other brain imaging methods like PET. FMRI uses strong magnetic fields to detect the magnetic signal from hydrogen atoms in water molecules in the brain. Changes in the magnetic properties of oxygenated versus deoxygenated hemoglobin allow FMRI to detect brain activity based on the blood oxygenation level dependent (BOLD) response to neural activation.