Final answer:
The neuroimaging method that uses a harmless radioactive substance is Positron Emission Tomography (PET), which tracks a radioactive tracer in the bloodstream to map brain or organ activity.
Step-by-step explanation:
The neuroimaging method that involves the use of a small amount of a harmless radioactive substance is Positron Emission Tomography (PET). In a PET scan, a mildly radioactive tracer is introduced into the bloodstream. Once it reaches the brain or another target organ, it enables the monitoring of blood flow and physiological activities, which correlates with the level of activity in different areas.
While MRI (Magnetic Resonance Imaging) and CT (Computed Tomography) provide excellent structural images, a PET scan offers insights into how parts of the patient's body function at a metabolic level. PET scans have been instrumental in diagnosing conditions such as Alzheimer's disease due to their ability to reveal functional abnormalities before structural changes become apparent.