Magnetic resonance imaging MRI...
Very, very strong magnetic field is produced around a patient. The nuclei of hydrogen atoms of water molecules line up with the magnetic field's alignment. Radio waves are pulsed at the patient and the aligned nuclei de-align and then realign. In the process they nuclei produce their own faint radio waves. These radio waves are picked up by sensors in the machine and are interpreted by a comuter to form an image.
This is good for looking at soft, squishy, water-rich organs.
FUnctional MRI detects blood oxygen and reveals how much oxygen is present in a part of the body. Low oxygen - low blood.